Woombies are for swaddling babies. Now, regular baby swaddles look like sleeveless sleep sacks with angel wings. Not only do they look cute, but the wings wrap around to pin your arms to your sides. Takes about five minutes to put one on properly -- and you can get your arms free in five seconds. (Plain ol' swaddling blankets are even better. I can get out of one of those before it's even completely on.)
Woombies, on the other hand, are stretchy sleeping bags that zip onto you in seconds and you can't break out of ever. Plus, you look like a little writhing worm with a baby head. How humiliating. On top of that, they don't have any velcro to rip up all the other stuff in the wash. No fun at all.
There are two versions of Woombies, the old ones that zip up from the bottom, and the new ones that zip down from the top. The old ones are easier to get on, but I can push one arm out the neck if I try hard. The new ones are a little trickier to put on, but even more secure.
Mom and Dad swear by Woombies, so you should do your best to make sure your parents don't get you one.