What does Google Backup?

I have a Galay Nexus and plan to flash over to a Takju or Yakju build. Yet to do that i also need to unlock my boatloader which wipes all my data. Now in settings it offers to "Back up my data" now what exactly does it back up. Does it back up like the iPhone in that it takes every single thing or is it more limited.

When I wipe the phone I want to be able to retrieve things like text messages and my app data.