dual exhaust question (kind of a bone head question)

I am trying to figure out the diffs between a h-pipe and a catback system. Are they used in conjunction or 2 separate technologies. Is there a advantage to using cat-back?....I know this is probably something I should know but I dont, and I aint afraid to ask the question that may get me laughed at....

As always insight is appreciated.:nice:

Oh and I did search and even googled, but all I got was info about the makers. not an explanation of what it is...
 
Sometimes cat-backs are more correctly termed 'after-cat systems'. That might make it seem more clear.