It is one of the coolest shows ever on TV. I love 24. I just wait for the seasons to come out on DVD though, so I can watch them all at once, instead of having to wait week-to-week.
If you haven't seen 24, and are a fan of action/thriller stuff, you should run, not walk, to the nearest video store and pick up the first season to watch. Don't watch them out of order, though. Totally worth the time and money.
As for what it's about, kind of hard to sum up. Just go watch it.