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 new QUARTILE.INC and QUARTILE.EXC functions both find a requested quartile of a supplied data set.
The difference between these two functions is that the Quartile.Inc function bases its calculation on a percentile range of 0 to 1 inclusive, whereas the Quartile.Exc function bases its calculation on a percentile range of 0 to 1 exclusive.
The Excel QUARTILE.INC function returns a requested quartile of a supplied range of values, based on a percentile range of 0 to 1 (inclusive).
The function is new in Excel 2010 and so is not available in earlier versions of Excel. However, Quartile.Inc is simply a renamed version of the old Quartile function, that is available in earlier versions of Excel.
The format of the Quartile.Inc 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 Quartile.Inc function is closely related to the Excel Percentile.Inc function in that:
The spreadsheets below show examples of the Excel Quartile.Inc function used to calculate the quartiles of the set of values 0  6 (stored in cells A1A7 of the example spreadsheet).
Formulas:
 Results:

Note that in the above examples:
For further examples of the Excel Quartile.Inc function, see the Microsoft Office website
If you get an error from the Excel Quartile.Inc 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. 