Need clarification on a noob question.
 

What is the biggest difference between a header back exhaust with cat and a cat back exhaust?

A header back is replacing the exhaust from the header....back.
A cat back is replacing the exhaust from the cat....back

Headerback means that everything after the header is replaced, and cat back means everything after the cat is replaced. So the difference is that the header back will not only replace your muffler and the pipe after the cat, it will also replace the over pipe right after the header and the front pipe.

A significant amount of horsepower gain versus very little horsepower gain. The front half of the exhaust is the most restrictive. The back half more or less is for sound.

Hopefully this diagram helps illustrate what everyone above is saying.

Headerback: Over Pipe + Front Pipe + Midpipe + Muffler
Catback: Midpipe + Muffler

http://blog.perrinperformance.com/wp...st-diagram.jpg

no one understand his question. The difference is that the aftermarket cat on a headerback (with cat) exhaust is in most cases a highflow Cat = more HP than a catback exhaust without touching the sotck Cat (more restrictive)

Whoa so I want a complete header back system then... Is this still legal in ca? Also how much more usually is it for header back vs cat back? And does header back void my warranty?

I don't recall any of them having CARB approval yet. I'm not from cali, so some others here might have more accurate information.

Voiding warranty will depend on the service tech you get. It would be difficult for them to prove replacing your exhaust damaged whatever part you're trying to get replaced under warranty- but if they make that kind of claim the onus is on you to prove them wrong afaik.