COUNTIF function in Excel 2007
You can count the number of cells in an Excel worksheet range based on certain criteria. Therefore this function provides power and utility. The range can be one or many cells.
The syntax is: countif(range, criteria). For example, ‘A2:A10’ is a range from which you can count values based on criteria. The criteria could be a number, for example, ‘>90’ or text like ‘yes’ or ‘no’ or any other text. You could count for example, all employees getting a salary of greater than 10000 per month or you could count all employees who opted for ‘insurance’. You could count ‘items’ in an invoice based on a category like ‘electrical’ or ‘electronic’. The usage possibilities are based on your problem that requires a quick solution. In fact, you can use wild cards. Let’s say you wish to count all employees or persons whose name begins with ‘N’. you could write the countif function as ‘=countif(Range of cells ‘,”N*”). Now if you find too many persons you can use the wild card ‘?’. The ‘*’ counts everything but the ‘?’ represents only one character. So if typed ‘N????’ in the criteria you would definitely catch ‘Nancy’ if it existed in your specified Excel cell range. The Human resources department of your company may use the countif function to count how many people left the company during a certain period and how this disturbed the marketing efforts of your company! Note: The formula for finding the empty cells in a column are, for example: ‘=countif(B2:B10,”<>”&”*”)

Further reading:
COUNTIF function