What is a design pattern and why are they useful?

Design patterns are reusable solutions to common software design problems. They provide proven templates, not code, that can be adapted to specific situations. Categories: Creational (object creation - Singleton, Factory), Structural (object composition - Adapter, Decorator), Behavioral (object interaction - Observer, Strategy). Benefits: shared vocabulary, proven solutions, code maintainability, easier communication among developers. Popularized by the Gang of Four (GoF) book.