Count[list, pattern] gives the number of elements in list that match pattern.
Count[expr, pattern, levelspec] gives the total number of subexpressions matching pattern that appear at the levels in expr specified by levelspec.
Level specifications are described in Section A.3.6.
See The Mathematica Book: Section 1.8.5 and Section 2.3.2.
See also: FreeQ, MemberQ, Cases, Select, Position.
Related package: Statistics`DataManipulation`.