I am constantly hearing to contract my abs or tuck them in whenever doing any excersice, but what exactly do I do? Is it like the vacuum exercise where you exhale completely?
Thanks

Suck your stomach in.

just keep them tight because if you suck them in you won't be breathing right. Stand straight, keep your shoulders back and your chest forward, usually help correcting your form and keeping your abs tight

I really learned alot about abs from that Special K pilates DVD that was out a while ago. I also find that knowing the anatomy of the torso helps. If you can find a nice pencil drawing of the muscles in your tummy, have a good look. Then lay on your back and see if you can find them. With your hands placed where they should be ... set them and feel with your hands. And like Jigglybits said, make sure you can breath with them set. With a bit of practice, you'll be able to set them even when you're vaccuuming etc.
Did that make sense?

When performing exercises, the abs should be contracted about 30%. If you suck them in completely, as in the ab vaccum, you can't breathe properly and can't focus on the one you are doing.

During the day, try to keep your abs in about 30% also. This really helps counteract the tendency for it all to hang out.

What my pilates teacher says is "zip and hollow" it's a good thing to say (in your head if your at the gym) before you do any exercises. You are drawing your navel to spine, that's the mental picture we use. We breathe in first, then start tightening the abs, much the same way as you exhale on effort before lifting a weight.

I always think of it as if my navel were a mattress button. Pull the mattress button in tightly to your spine and your abs are engaged.