Something to clarify is that iPhoto, Aperture and Lightroom are not editing applications... they are designed to organize your images, and they also do some editing. They are also RAW converters.
That said, Aperture and Lightroom can do about 85% to 95% of the editing tasks that most photographers need, and they do it well. But there are some fundamental differences between Ap and Lr and editing applications like Photoshop (full, or the lite version Elements).
iP, Ap and Lr edit non-destructively. When you import an image these applications make a note of where of the image is. From then on all the edits are simply recorded as notes in a database record. The original file is never touched. The edited image is not created until you either export or print it. This means you can go back to the original file at any time.
The downside to iP, Ap, and Lr is that you can't work in layers, nor can you edit at the pixel level.
Ps (full and Elements) plus other editing suites are not "non-destructive"... that is, the default behaviour is to replace pixels in the image as you edit (you need to do a "save as" to preserve the original pixels. Ps (et al) also let you work with layers, and to get down to the individual pixels. Most photographers with iP, Ap, or Lr will also have an external editor for those images that need serious editing.
Hope that helps. Welcome to world of pixel pushing.