Can someone explain how Corps are taxed? 

I have seen a client in 2 years report $150k total in losses. The 3rd year (this year) they made $100k. The tax due says $0. If I increase the $100k to let's say $160k ($10k more than all the losses they've accumulated) then they start to owe taxes. Is that normal? Do corporations losses carryover forever until they finally generate an overall profit?