NorCalwrx

catback is from the last cat up(basically the muffler and a resonator) turbo back is from the turbo up (downpipe, midpipe, muffler, cats etc.)

Wrxneffect

iTrader: (7)

turboback

DLUX WRX

iTrader: (1)

yeah, a turbo back will give your more gains.
a trubo back includes a down pipe and mid pipe (which are usually sold together as one unit called a down pipe) and a cat back section. this will yeild the most power. a cat back will give you a lilttle more power but the stock down pipe should be changed to get max power out of your car. the stock down pipe is very restrictive. most after market down pipes have no cats, so if you want to go that way, know that you will be esentially running a straight exhaust system.

Ditty77

iTrader: (1)

If you go with a turboback, you should also go with an uppipe, header etc. but cash doesn't always allow that.

I have FULL aftermarket exhaust and it is an incredible difference.
No complaints. (except o2 CEL but I'll fix that soon)

gordy

iTrader: (1)

catback = 3 parts
turboback = 4 parts

catback + downpipe = turboback*

*since any downpipe you get will have the midpipe integrated.. unless you get one that doesn't replace the midpipe (no why would you do that?)