FTL: AE is free expansion to FTL which includes a new alien race along with new events, weapons, playable ships, drones, and more! It also adds additional musical tracks by Ben Prunty, and events by Tom Jubert and guest writer Chris Avellone.
If you own FTL on Steam it should automatically update the game to the newest version, v1.5.10. If you instead own a DRM-Free version of FTL, you need to re-download the game from the original location and then re-install it. The following link has more information about updating DRM-Free versions - Update Instructions
Additionally, FTL has also been released on iPad! It is the complete game (same as PC) and includes the new AE content. It costs $10 in the US Store and is available on the App Store worldwide. Note: FTL purchases on computer cannot be transferred or redeemed on iPad.