The Skew of a data set is a measurement of the asymmetry of the distribution about the mean.
Examples of positive and negative skewed distributions are shown in the charts below:
Positive Skew  Negative Skew 
The Excel SKEW function calculates the skewness of the distribution of a supplied set of values.
The format of the function is:
Where the number arguments provide a minimum of 3 values that make up the data set. These can be individual numbers or arrays of numbers and can be supplied to the function either directly or as a reference to a range of cells containing values.
In the latest versions of Excel, you can input up to 255 number arguments to the Skew function, but in Excel 2003, you can only input up to 30 arguments.
A  B  

1  1  
2  1  
3  2  
4  2  
5  2  
6  2  
7  3  
8  3  
9  3  
10  4  
11  4  
12  5  
13  6  
14  7  
15  8 
Column A of the above spreadsheet on the right shows 15 data values. The distribution of these values is shown in the chart below:
The skewness of the values in the spreadsheet can be calculated using the Excel Skew Function as follows:
This gives the result 0.863378312, indicating that the data set has a positive skew.
Further information and examples of the Excel Skew function can be found on the Microsoft Office website.
If you get an error from the Excel Skew function this is likely to be one of the following:
#DIV/0!   
Occurs if either:

#VALUE!    Occurs if any of the supplied number arguments that are supplied directly to the Skew function are not recognised as numeric values. 
The Skew function also returns an error if any of the input arguments are errors. 