The ISERROR function is useful for testing the outcome of a formula so that you can account for any errors. The syntax is:

=ISERROR(C2) 

C2 is the cell you want to test for an error. If C2 contains #N/A, #VALUE!, #REF!, #DIV/0!, #NUM!, #NAME!, or #NULL!, the result will be TRUE, otherwise it will be FALSE.

The most common use for this function is within the IF function to check for error and then handle it. For example:

=IF(ISERROR(C2),C1,B1) 

The formula above checks cell C2 for an error, if there is an error it takes the value in C1, otherwise the formula will use the value in B1. This is also useful within the VLOOKUP function since non matched values will return an error.