One neural net is creating counterfeit bills and passing them off to a second neural net, which is trying to distinguish the real money from the fakes. Result: two neural nets that are better than either one would have been without the competition.

Relevant links:

http://arxiv.org/pdf/1406.2661v1.pdf

http://arxiv.org/pdf/1412.6572v3.pdf

http://soumith.ch/eyescream/