Computer Science Glossary 📖
Plain-language definitions for 50+ essential CS terms — curated for GSEB, CBSE, ICSE & BCA students in Surat. Each term includes an answer-engine-friendly explanation and a practical example.
Jump to: OOP Concepts · Java · Python · SQL & Databases · Data Structures · Networking · Web / HTML
OOP
What is OOP (Object-Oriented Programming)?
OOP is a programming paradigm that organises software around objects — bundles of data (attributes/fields) and behaviour (methods). Instead of writing a sequence of instructions, you model real-world entities.
The four pillars:
| Pillar | Meaning | Quick Example |
|---|---|---|
| Encapsulation | Hide internal data; expose only a clean interface | private int balance; in a BankAccount class |
| Abstraction | Show only what is necessary; hide complex internals | A Car class exposes drive() but hides fuel injection logic |
| Inheritance | Child class reuses properties/methods of a parent class | Dog extends Animal |
| Polymorphism | Same method name, different behaviour depending on the object | animal.speak() returns "Woof" for Dog, "Meow" for Cat |
Board relevance: All four pillars are examined in GSEB Std 12, CBSE Class 12, and ISC Paper 1.
What is a Class?
A class is a blueprint or template that defines what attributes (data) and methods (behaviour) an object will have. A class itself holds no data — it is just the definition.
// Blueprint: no actual student exists yet
class Student {
String name;
int rollNo;
void display() {
System.out.println(name + " - " + rollNo);
}
}
What is an Object?
An object is a specific instance of a class — a concrete entity with actual values.
Student s1 = new Student(); // Object created from the Student class
s1.name = "Riya";
s1.rollNo = 42;
s1.display(); // Output: Riya - 42
Think of it this way: Class = Cookie Cutter, Object = Actual Cookie.
What is Inheritance?
Inheritance allows a child (sub) class to acquire the properties and methods of a parent (super) class, enabling code reuse.
class Animal {
void breathe() { System.out.println("Breathing..."); }
}
class Dog extends Animal { // Dog inherits breathe()
void bark() { System.out.println("Woof!"); }
}
Official reference: Oracle Java Inheritance Docs
What is Polymorphism?
Polymorphism means "many forms." The same method name can behave differently depending on the object calling it.
- Compile-time (Method Overloading): Same method name, different parameters in the same class.
- Runtime (Method Overriding): Child class provides its own implementation of a parent method.
What is Encapsulation?
Encapsulation is the practice of bundling data and the methods that operate on it inside a single unit (class), and restricting direct access to the data using access modifiers (private, protected).
Access is controlled through getters and setters.
What is Abstraction?
Abstraction means hiding the implementation complexity and exposing only the essential interface to the user.
In Java, abstraction is achieved using:
- Abstract classes (
abstractkeyword) - Interfaces (
interfacekeyword)
Java
What is JVM (Java Virtual Machine)?
The JVM is the runtime engine that executes Java bytecode. When you compile a .java file, the javac compiler produces platform-neutral .class bytecode. The JVM installed on the host machine then interprets and executes that bytecode.
Why this matters: This is what makes Java platform-independent — "Write Once, Run Anywhere."
Source Code (.java) → javac compiler → Bytecode (.class) → JVM → Machine Code
Official reference: Oracle JVM Specification
What is JDK vs JRE vs JVM?
| Term | Full Form | What it Contains |
|---|---|---|
| JDK | Java Development Kit | JRE + Compiler (javac) + Developer tools |
| JRE | Java Runtime Environment | JVM + Standard Libraries |
| JVM | Java Virtual Machine | The runtime engine only |
Rule of thumb: Developers install JDK. End users install JRE. JVM runs inside both.
What is a Constructor in Java?
A constructor is a special method that is automatically called when an object is created. It has the same name as the class and no return type.
class Book {
String title;
Book(String t) { // Constructor
title = t;
}
}
Book b = new Book("Java for Beginners"); // Constructor called automatically
What is an Interface in Java?
An interface is a contract that a class must follow. It declares method signatures but provides no implementation. A class that implements an interface must define all its methods.
interface Drawable {
void draw(); // No body
}
class Circle implements Drawable {
public void draw() { System.out.println("Drawing a circle"); }
}
Official reference: Oracle Java Interfaces Tutorial
What is Exception Handling in Java?
Exception handling is a mechanism to handle runtime errors gracefully so the program doesn't crash abruptly.
Key keywords: try, catch, finally, throw, throws
try {
int result = 10 / 0; // ArithmeticException
} catch (ArithmeticException e) {
System.out.println("Cannot divide by zero: " + e.getMessage());
} finally {
System.out.println("This always runs.");
}
What is Garbage Collection in Java?
Garbage Collection (GC) is Java's automatic memory management system. The JVM periodically identifies and removes objects that are no longer referenced by the program, freeing up heap memory.
Unlike C/C++, Java programmers do not manually allocate or free memory.
Python
What is Python?
Python is a high-level, interpreted, general-purpose programming language known for its readable syntax that resembles plain English. Created by Guido van Rossum and first released in 1991, it is widely used in web development, data science, AI/ML, and scripting.
Official reference: Python.org Documentation
What is an Interpreter vs a Compiler?
| Interpreter | Compiler | |
|---|---|---|
| How it works | Translates and executes code line by line | Translates entire program first, then executes |
| Example | Python, JavaScript | C, C++, Java (javac) |
| Error detection | Stops at first error | Reports all errors at once |
| Speed | Slower execution | Faster execution |
Python is interpreted. Java is compiled to bytecode (then JVM interprets it — a hybrid approach).
What is a List in Python?
A list is an ordered, mutable (changeable) collection of items. Items can be of different data types. Lists are defined with square brackets [].
marks = [85, 90, 78, 95]
marks.append(88) # Add item
print(marks[0]) # Access: 85
marks[1] = 92 # Modify
Official reference: Python Lists Documentation
What is a Dictionary in Python?
A dictionary is an unordered collection of key-value pairs. Keys must be unique and immutable; values can be anything.
student = {"name": "Priya", "grade": "A1", "marks": 98}
print(student["name"]) # Output: Priya
student["marks"] = 99 # Update
What is a Function in Python?
A function is a named, reusable block of code that performs a specific task. Functions are defined using the def keyword.
def greet(name):
return f"Hello, {name}!"
print(greet("Riya")) # Output: Hello, Riya!
What is self in Python?
self refers to the current instance of the class. It is always the first parameter in instance methods and allows you to access the object's attributes and methods.
class Student:
def __init__(self, name):
self.name = name # self.name = instance attribute
def display(self):
print(f"Student: {self.name}")
SQL
What is SQL?
SQL (Structured Query Language) is the standard language for creating, reading, updating, and deleting data in relational databases. It is used by MySQL, Oracle, PostgreSQL, SQLite, and SQL Server.
Official reference: W3Schools SQL Tutorial
What is a Primary Key?
A primary key is a column (or set of columns) that uniquely identifies each row in a table. Rules:
- Must be unique — no two rows share the same value.
- Must be NOT NULL — cannot be empty.
- Each table can have only one primary key.
CREATE TABLE Students (
StudentID INT PRIMARY KEY,
Name VARCHAR(100)
);
What is a Foreign Key?
A foreign key is a column in one table that references the primary key of another table. It enforces referential integrity — you cannot reference a row that doesn't exist.
CREATE TABLE Orders (
OrderID INT PRIMARY KEY,
StudentID INT,
FOREIGN KEY (StudentID) REFERENCES Students(StudentID)
);
What is a JOIN in SQL?
A JOIN combines rows from two or more tables based on a related column.
| JOIN Type | Returns |
|---|---|
INNER JOIN | Only matching rows in both tables |
LEFT JOIN | All rows from left table + matching rows from right |
RIGHT JOIN | All rows from right table + matching rows from left |
FULL JOIN | All rows from both tables |
SELECT s.Name, o.OrderID
FROM Students s
INNER JOIN Orders o ON s.StudentID = o.StudentID;
What is Normalization in SQL?
Normalization is the process of organizing a database to reduce data redundancy and improve data integrity. It involves dividing a large table into smaller, logically related tables.
| Normal Form | Rule |
|---|---|
| 1NF | Each column has atomic (indivisible) values; no repeating groups |
| 2NF | 1NF + No partial dependency on composite keys |
| 3NF | 2NF + No transitive dependency |
What is DDL vs DML vs DCL?
| Category | Full Form | Commands | Purpose |
|---|---|---|---|
| DDL | Data Definition Language | CREATE, ALTER, DROP, TRUNCATE | Define structure |
| DML | Data Manipulation Language | SELECT, INSERT, UPDATE, DELETE | Manipulate data |
| DCL | Data Control Language | GRANT, REVOKE | Control access |
Data Structures
What is an Array?
An array is a linear data structure that stores a fixed-size, sequential collection of elements of the same data type. Elements are accessed using an index starting at 0.
int[] marks = {85, 90, 78, 95};
System.out.println(marks[0]); // 85
Arrays are contiguous in memory, making random access O(1).
What is a Stack?
A stack is a linear data structure that follows the LIFO (Last In, First Out) principle — the last element added is the first to be removed.
Operations: push (add), pop (remove), peek (view top)
Real-world example: A stack of plates — you take from the top, you add to the top.
Used in: Function call stack, expression evaluation, backtracking algorithms.
What is a Queue?
A queue is a linear data structure that follows the FIFO (First In, First Out) principle — the first element added is the first to be removed.
Operations: enqueue (add to rear), dequeue (remove from front)
Real-world example: A ticket queue at a cinema — first in, first served.
What is Recursion?
Recursion is a technique where a function calls itself to solve a smaller version of the same problem. Every recursive function must have:
- Base case — a condition that stops the recursion.
- Recursive case — the function calling itself with a smaller input.
def factorial(n):
if n == 0: # Base case
return 1
return n * factorial(n - 1) # Recursive case
print(factorial(5)) # 120
What is a Linked List?
A linked list is a dynamic data structure where each element (called a node) contains:
- Data — the stored value
- Pointer/Reference — a link to the next node
Unlike arrays, linked list nodes are not stored contiguously in memory. Insertion and deletion are O(1) at the head; searching is O(n).
Networking
What is an IP Address?
An IP (Internet Protocol) address is a unique numerical label assigned to every device connected to a network. It identifies the device's location on the network.
- IPv4: 32-bit address (e.g.,
192.168.1.1) - IPv6: 128-bit address (e.g.,
2001:0db8:85a3::8a2e:0370:7334)
What is HTTP vs HTTPS?
| HTTP | HTTPS | |
|---|---|---|
| Full form | HyperText Transfer Protocol | HTTP Secure |
| Security | Unencrypted | Encrypted using TLS/SSL |
| Port | 80 | 443 |
| Usage | Older/internal sites | All modern websites |
What is a Protocol?
A protocol is a set of rules and conventions that define how data is transmitted and received over a network. Examples:
- HTTP/HTTPS — Web pages
- FTP — File transfer
- SMTP — Email sending
- TCP/IP — Core internet communication
What is DNS (Domain Name System)?
The DNS is the internet's "phone book." It translates human-readable domain names (like vishnudigital.com) into IP addresses (like 185.199.108.153) that computers can use to locate servers.
Web
What is HTML?
HTML (HyperText Markup Language) is the standard language for creating the structure and content of web pages. It uses tags (enclosed in <>) to mark up text, images, links, and other elements.
Official reference: MDN Web Docs — HTML
What is CSS?
CSS (Cascading Style Sheets) is the language used to style and visually format HTML elements — controlling colours, fonts, layout, spacing, and responsiveness.
Official reference: MDN Web Docs — CSS
What is JavaScript?
JavaScript is the programming language of the web. It runs in the browser and makes web pages interactive — responding to user actions, updating content without reloading, and communicating with servers (AJAX/Fetch).
Official reference: MDN Web Docs — JavaScript
What is the DOM (Document Object Model)?
The DOM is a programming interface for HTML documents. It represents the page as a tree of nodes (elements, attributes, text) that JavaScript can read and modify dynamically.
document.getElementById("heading").textContent = "New Title";
Summary
This glossary covers 50+ terms across OOP, Java, Python, SQL, Data Structures, Networking, and Web technologies — aligned with GSEB, CBSE, ICSE, and BCA curricula.
For deeper dives, explore the full tutorials: