The Engineer class is a defensive role, primarily, able to erect turrets, ammo/health dispensers and teleports to support his team (and then whack them repeatedly with a wrench to speed up build times).
As with a lot of TF2 classes, however, he's also versatile enough to be useful anywhere on the battlefield. Escorted to an advanced control point by tougher comrades, his teleports and other contraptions can change the course of a battle. Turrets, in particular, cause huge problems for the opposition until they're destroyed.
For more on TF2 (loads more), check out the Team Fortress 2 PC gamepage. It's also due out on 360 and PS3 as part of The Orange Box, which launches on 10th October on Steam and 12th October at retail for PC and 360.