The Excel Countif function returns the number of cells (of 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 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.
eg. the condition "A*e" will match all cells containing a text string beginning with "A" and ending in "e".The criteria can be either:
  a numeric value (which may be an integer, decimal, date, time, or logical value) (eg. 10, 01/01/2008, TRUE)  
or  
  a text string (eg. "Text", "Monday")  
or  
  an expression (eg. ">12", "<>0") 
and can be supplied to the function either directly, as a reference to a cell containing the criteria, or as a value returned from another function or formula.
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 considered to be 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, numerical values, dates or logical values in the data spreadsheet on the right:
The format of the functions is shown in the lower left spreadsheet and the results are shown in the lower right spreadsheet.
Formulas:

Results:

The example below shows the Excel Countif function used to identify duplicates in a column containing reference numbers. Note that the function in this example is written so that it highlights only the second, third, etc instance of a duplicate value.  It does not highlight the first instance of the value.
The function works by counting the number of times the reference number in column A of the current row has occurred so far.
Absolute and Relative cell references are used in the definition of the supplied array, (defined as A$2:A2 in the first cell). This ensures that, as the function is copied down to subsequent rows, the array, always refers to the cells of column A, up to the current row (ie. A$2:A3 in row 3, A$2:A4 in row 4, etc)
As the function only searches rows up to and including the current row, the result will be 2 (or greater) only for repeated values.
Formulas:

Results:

In the above example the Excel COUNTIF function has, as expected, identified the duplicate reference in cell A5
For a further example of the COUNTIF function being used to identify duplicates, see the Excel Duplicates page of this site. The Microsoft Office website also gives more examples of this function.