Design patterns are reusable solutions to common problems that occur during software design and development. They are like templates or blueprints that provide a structured way to solve recurring design challenges in a software application. These patterns are not specific to a particular programming language or technology; instead, they offer general guidelines and best practices for designing robust, maintainable, and efficient software.

There are mainly 3 categories of design patterns introduced by the "Gang of Four" (GoF) in their book "Design Patterns: Elements of Reusable Object-Oriented Software." explained below:

Creational Patterns: These patterns deal with object creation mechanisms, abstracting the instantiation process.

• Abstract Factory
• Builder
• Factory Method
• Prototype
• Singleton

Structural Patterns: These patterns focus on how objects are composed or structured to form larger, more complex structures.

• Adapter
• Bridge
• Composite
• Decorator
• Facade
• Flyweight
• Proxy

Behavioral Patterns: These patterns are concerned with how objects interact and communicate with one another.

• Chain of Resp.
• Command
• Interpreter
• Iterator
• Mediator
• Memento
• Observer
• State
• Strategy
• Template
• Visitor