Back Crosses ( can someone show me an example)

[nnnnnn]

I am not clear as to what a back cross is, can someone please show me an example. I know that you cross a F1 offspring with its parent, but what is a backcross trying to prove?

[Mr Skeptic]

http://www.britannica.com/EBchecked/topic/48062/backcross

"the mating of a hybrid organism (offspring of genetically unlike parents) with one of its parents or with an organism genetically similar to the parent. The backcross is useful in genetics studies for isolating (separating out) certain characteristics in a related group of animals or plants."