The 1st, 2nd, 3rd or 4th quartile of a range of data is the value that 25%, 50%, 75% or 100% (respectively) of the data values fall within.
This is explained in more detail on the Wikipedia Quartile pageIn Excel 2010, the QUARTILE function has been renamed the QUARTILE.INC function.
Although it has been replaced, the Quartile function is still available in Excel 2010 (stored in the list of compatibility functions), to allow compatibility with earlier versions of Excel.
The Excel QUARTILE function returns a requested quartile of a supplied range of values.
The format of the function is:
Where the function arguments are:
array    The range of data values for which you want to calculate the specified quartile. 
quart    An integer between 0 and 4, representing the required quartile. 
If the requested quartile falls between two of the values in the supplied array, Excel interpolates between these values to calculate the quartile value.
Note that the Excel Quartile function is closely related to the Excel Percentile function in that:
The spreadsheets below show examples of the Excel Quartile function used to calculate the quartiles of the set of values 06 (stored in cells A1  A7 of the example spreadsheet).
Formulas:
 Results:

Note that in the above examples:
For further examples of the Excel Quartile function, see the Microsoft Office website
If you get an error from the Excel Quartile function this is likely to be one of the following:
#NUM!    Occurs if either:

#VALUE!    Occurs if the supplied value of quart cannot be interpreted as a numeric value. 