I got this from the GE webpage:

Thrust Reverser: Thrust reversers serve as an aircraft's main brakes
on landing. There are three types of thrust reversers: translating
cowl, clam shell and turboprop reverse pitch. All three literally
reverse the engines thrust by closing in when deployed by the pilot
pushing the air out the front of the engine rather than the back. This
motion decreases the speed of the aircraft and is the loud noise you
hear when landing.

I'm assuming you're mostly concerned with 'jets', then you would be
wondering about the clamshell and translating cowl. Clamshells when
activated deflect the exhaust forward, and the translating cowl only
sends the bypass air forward.

A couple of discussions on thrust reversal:
http://www.airliners.net/discussions...ead.main/54508
http://www.airliners.net/discussions...ead.main/43007

-lance smith