It's similar to the "Market" on an Android, and I'm sure iPhones have a store of some sort as well. You can use it to browse hundreds (if not thousands by now) of apps that you can install on your phone. Some free, some paid.
It would be nice if it wasn't so slow. I tried using it several times and it takes forever to do anything on it! Ill stick to BB app world and sites like crackberry. ATT must have some bugs with the new program.
Posted from my CrackBerry at wapforums.crackberry.com