TTL stands for Through-The-Lens and is basically your flash on the “Auto” setting. It automatically calculates the flash intensity necessary to properly light the subject, then fires when you depress the shutter button. I don’t actually have a flash with TTL so if someone else gives you a better description than I can, you’d better trust their word first. I only know what I have read about it.
There is an article describing the different kinds of TTL flash on exposureguide.com
I always use my flash on Manual since that’s all I’ve got 🙂 I set my camera to Manual, and set my exposure properly for ambient light, then power on the flash and do a few test shots to make sure it’s the right intensity. Of course so far, I haven’t done much action shooting with flash, so I’m not exactly an authority on the subject.