The Hulk is really Bruce Banner. While saving a teenager named Rick Jones from a gamma bomb testing (this is not Ultimates series Hulk), he got caught in the blast, and now when he gets angry he turns into that classic green (formerly gray) monster called Hulk.
In Heroclix, most Hulks earlier on grew in strength as they were hurt and got angrier, or at least had a click when they were supposed to be Dr. Banner. After about the first half of the Golden Age, they lost this; he is assumed to already be quite enraged. On December 14, 2011, he got his own theme, a feat only done by Superman and Batman besides.