What are 'Design Patterns' in software engineering?

Design patterns are proven templates for solving recurring design problems. Creational (object creation): Singleton (one instance — e.g., database connection pool), Factory, Abstract Factory, Builder, Prototype.  Structural (composing objects): Adapter, Bridge, Composite, Decorator, Facade.  Behavioural (object interaction): Observer, Strategy, Command, Template Method, Iterator. In banking Singleton pattern is used for connection pools, Observer for transaction notifications and Strategy for different interest calculation algorithms.