What are macros in Rust and how do they differ from functions?

Macros in Rust: Metaprogramming - code that writes code, compile-time code generation, pattern matching on syntax, hygiene prevents variable capture. Types: Declarative macros (macro_rules!) and Procedural macros. Differences from functions: Operate on syntax not values, variable arguments, expanded at compile time, can generate any valid Rust code, no runtime overhead. Example: macro_rules! vec creates vectors at compile time.