Pushdown automata is a finite automata with extra memory called stack which helps pushdown automata to recognize Context Free Languages.
A pushdown autimaton is a way to implement a context-free grammar in a similar way we design DFA for a regular grammar. A DFA can remember a finite amount of information, but a PDA can remember an infinite amount of information.
Basically a pushdown autimaton is-
"Finite state machine" + "a stack"
A pushdown autimaton has three components-
an input tape,
a control unit, and
a stack with infinite size.
A pushdown automata (PDA) can be defined as:
Q:is a set of states
q0:is the initial state
Z: is the initial pushdown symbol
F: is the set of final states etc.