Explain the Decorator pattern and when to use it.

Decorator pattern attaches additional responsibilities to objects dynamically without modifying original class. Wraps objects with decorator classes implementing same interface. Examples: Java I/O streams (BufferedReader wrapping FileReader), adding logging/caching to services, UI component borders/scrollbars. Benefits: more flexible than inheritance, follows Single Responsibility, supports Open/Closed principle. Multiple decorators can be stacked. Different from inheritance - decoration is at runtime, inheritance at compile time.