Aperture vs Photoshop
I'm currently making the transition from Lightroom over to Aperture. You are right in that Aperture is more like Lightroom than Photoshop. Its primary use is as a digital library for managing your photos, and videos. Much like iPhoto.

Lightroom and Aperture both have good editing functionality and both have the ability to use plugins for expanding the editing functions.

Both applications also allow non-destructive editing. Meaning you can always go back to the original image no matter what changes you do to the image within the application.

Aperture, like Lightroom, is designed with the photographer in mind. Digital assest management, photo editing, and exporting abilities for printing etc...

It's best to use either (or both) of these alongside Photoshop. I use Photoshop when it comes down to the heavy editing, or when I"m trying to do something more creative with the image outside the abilities of the other two applications.

I'm still learning Aperture so I cannot comment too much on its strengths and weaknesses vs Lightroom. Here is a post by Scott Bourne as to why he uses Aperture over Lightoom.

Hope some of what is here helps.