It is a computational model that can be used to recognize context-free languages that are not context-sensitive. It is a state machine and has the memory of one infinite stack. It is used in the Theory of Computation and for designing compilers.

Pushdown Automata (PDA)

Push Down Automata (PDA's) are state machines that accept a language. A PDA is said to accept a Language (L) if and only if (L) is Context Free.

A PDA is similar to a Deterministic Finite Automata with the exception that the PDA carries a stack. When parsing a given char in an input string, the PDA not only has the option of switching states, but also has the ability to push or pop a char from the stack. This stack makes the PDA more powerful than the DFA.

Without a stack the DFA is only able to accept languages if and only if they are Regular.

A PDA is a language accepter. A Context Free Grammar is a language generator that generates the same class of languages accepted by PDA's.