The Excel Countif function returns the number of cells within a supplied range, that satisfy a given criteria.
The syntax of the function is:
where the function arguments are as follows:
range    The range of cells that should be tested against the supplied criteria and counted if the criteria is satisfied. 
criteria    A userdefined condition that is tested against each of the cells in the supplied range. 
Wildcards
You can also use the following wildcards in textrelated criteria:
?  matches any single character
*  matches any sequence of characters
if you do actually want to find the ? or * character, type the ~ symbol before this character in your search.
E.g. the condition "A*e" will match all cells containing a text string beginning with "A" and ending in "e".The supplied criteria can be either:
Note that if your criteria is a text string or an expression, this must be supplied to the function in quotes.
Also note that the Excel Countif function is not casesensitive. So, for example, the text strings "TEXT" and "text" will be evaluated as equal.
A  B  C  D  

1  Sunday  07Sep2008  0  TRUE 
2  Monday  08Sep2008  2.1  TRUE 
3  Wednesday  10Sep2008  2  TRUE 
4  Thursday  11Sep2008  3  FALSE 
5  Wednesday  17Sep2008  2.5  FALSE 
6  Tuesday  23Sep2008  3  FALSE 
7  Wednesday  24Sep2008  6  FALSE 
8  Sunday  05Oct2008  4  FALSE 
9  Saturday  11Oct2008  0  FALSE 
The following example shows the Excel Countif function used to count cells containing text strings, numeric values, dates or logical values within the above data spreadsheet on the right.
The functions and the results are shown in the spreadsheets below.
Formulas:

Results:

In the following example, the Excel Countif function is used to identify duplicates in a column containing reference numbers. The function works by counting the number of times the reference number in column A of the current row has occurred so far.
Note that the function in this example uses a combination of relative and absolute cell references, so that, as the formula in cell B2 is copied down column B of the spreadsheet, the reference to A$2:A2 is automatically updated to A$2:A3, A$2:A4, A$2:A5, etc.
This ensures that only the repeated instances of a duplicate value are highlighted. I.e. The function does not highlight the first instance of a value.
Formulas:

Results:

The above 'Results' spreadsheet shows that the Excel Countif function has, as expected, identified the duplicate reference in cell A5 of the spreadsheet.
For further examples in which the Countif function is used to identify duplicates, see the Find Excel Duplicates and Remove Excel Duplicates pages of this site. The Microsoft Office website also provides further examples of this function.
If you get an error from the Excel Countif Function, this is likely to be the #VALUE! error:
#VALUE!    Occurs if the supplied criteria argument is a text string that is greater than 255 characters in length. 