Cases

Cases[{e1, e2, ...}, pattern] gives a list of the ei that match the pattern.

Cases[{e1, ...}, pattern -> rhs] gives a list of the values of rhs corresponding to the ei that match the pattern.

Cases[expr, pattern, levelspec] gives a list of all parts of expr on levels specified by levelspec that match the pattern.

Examples

Find all integers:

Cases[{1, a, 2, b, 3}, _Integer]
(* {1, 2, 3} *)

Extract matching parts with transformation:

Cases[{f[1], g[2], f[3]}, f[x_] -> x]
(* {1, 3} *)

Please visit the official Wolfram Language Reference for more details.