I was buying a new brakeset and found a good deal on a caliper set, but the guy says he bought them and doesnt know for sure if it is a front and rear set. I was wondering what the difference between a front and rear caliper was? It is an ultegra set if that changes anything.

First, front and rear brakes require different length attachment bolts and many brakes, such as dual pivot are modulated differently.

I was going to say location but rando is a little more acurate.

Front will have a longer attachment bolt - that's how I pick them. It has to go through the fork crown, but the rear only has a much smaller part of the frame to bolt on to and therefore a shorter bolt.