Someone explain Enterprise Firewalls to me

I'm doing some research and google hasn't been too much help as there seems to be a lot of misleading marketing information. As I understand it, there have been four generations of firewalls that have built on top of each other (1st-4th Gen). Then essentially what is available now are Next Generation Firewalls or Unified Threat Management Systems. Are those essentially what would be known as Enterprise Firewalls, or is that a completely different type of product.

Sorry for the ignorance, but I figured this would be the right place to ask such a question. If someone could explain it to me in grade school terms, that would be awesome.