The Exclusive Or logical operation returns True if one (and only one) of two supplied conditions evaluate to True. It can be thought of as "either A or B, but not both A and B".
More generally (i.e. when there is just one condition or when there are more than two conditions), the Exclusive Or operation evaluates to True if an odd number of conditions evaluate to True.
For further information on the Exclusive Or logical operation, see the Wikipedia Exclusive Or Page
The Excel Xor function returns the Exclusive Or logical operation for one or more supplied conditions.
I.e. the Xor function returns TRUE if an odd number of the supplied conditions evaluate to TRUE, and FALSE otherwise.
The syntax of the function is:
where the logical_test arguments are between 1 and 254 supplied conditions that evaluate to either TRUE or FALSE.
Note that:
The following spreadsheet shows six examples of the Excel Xor function.
Formulas:

Results:

For further information and examples of the Excel Xor function, see the Microsoft Office website.
#VALUE!    Occurs if none of the conditions supplied to the Xor function evaluate to logical values or numeric values. 
#NAME?    Occurs if Excel does not recognise the function name. This is probably because you have an earlier version of Excel, which does not support the Xor function. 