^^In other words...
Apple puts a lot of restrictions on the apps that make it into the App Store and so the iPhone community miss out on a lot of really neat things. Jailbreaking opens the door to these really cool apps and lets you run anything on your iPhone (no restrictions whatsoever.. if you have the files for it.. it'll run)
It lets you do things like :
-Record Video on the 2G and 3G
-Add themes to your iPhone (make ANY part of your iPhone look exactly how you want it to)
-Enable MMS on the 2G
-Download Youtube videos (Yourtube FTW! Pumpkin is awesome

)
-Lots of little (or big) tweaks to the phone that make it so much more awesome
and SOOOO much more!
And there is no downside except it kills your warranty with Apple, but a restore in iTunes will bring the warranty back to life
