It's a professional Photo app from Apple. Shortly described, it's an advanced version of iPhoto. It got advanced editing and storage capabilites.
Here's the Apple site for it.
Here is a screenshot from my Aperture library (just got it).
It's really awesome, and it runs on any Intel/G5 system with 1.8 Ghz or more and 1 GB RAM.
Runs flawless on my 2.0 GHz Core Duo MacBook with 1 GB RAM.
Know what, screw my screenie. Takes 500 years to upload, and 4 other people probably beat me to it already.