From Help:
Counts the number of cells within a range that meet the given criteria.
Syntax
COUNTIF(range,criteria)
Range is the range of cells from which you want to count cells.
Criteria is the criteria in the form of a number, expression, or text that
defines which cells will be counted. For example, criteria can be expressed
as 32, "32", ">32", "apples".
=COUNTIF(A1:A100,B1)
will count the number of time the value in B1 appears in A1 through A100
