Hierarchy classifying formal languages by the restrictions on their grammars and the automata required to parse them. Type 3 (regular) — regular grammars, parsed by finite-state machines; all finite languages are regular. Type 2 (context-free) — single non-terminal on the left, parsed by pushdown automata; deterministic and non-deterministic forms have different expressive power; covers parenthesis matching. Type 1 (context-sensitive) — more than one symbol on the left, parsed by linearly-bounded automata (Turing machines with finite tape — closest to real computers). Type 0 (recursively enumerable) — unrestricted grammars, parsed by Turing machines; equivalent in power to lambda calculus and general recursive functions.