Aperture is a database. It's oddest feature is that "photo" is close to meaningless. The Images you see are created on-the-fly by reading the Original (which is _never_ altered) and a text file of instruction (called, confusingly, a Version) which are then applied to create the Image. This image is often referred to as (I told you it was confusing) a Version.
You changes are always and automatically saved. Any adjustment you make or metadata you add or change is permanent until you change it again.
You don't create _files_ until you need them outside of Aperture. "Export" in Aperture, means "make me a file I can use with other programs".
Here's a primer I penned.