Aperture is a digital asset management tool with an excellent user interface. It is also excellent for basic image adjustments. It is NOT Photoshop... but then, it doesn't really need to be. Most people using Photoshop barely scrape the surface of it's capabilities when they do their day-to-day photo adjustments.
LightRoom came along sometime after Apple developed Aperture. For the most part, LightRoom is playing catchup with Aperture, though they do have a perceived edge with local image adjustments. I suspect the next version with Aperture will once again remove any doubt which of the two is the pro application.
Regardless, if you are a serious photographer (pro or hobbyist) you should be using Lightroom or Aperture first... think of these programs as an "operating system" for photographers. Once you thoroughly understand the programs and how to truly manage your workflow, you'll see how each can handle 99% of all your needs.
I shoot about 50,000, - 75,000 images per year... I rarely use Photoshop anymore, Aperture has made my life much simpler and much more organized. From importing images into the system, managing IPTC data, Keywords, sorting, ranking, image adjustements and ultimately exporting to whatever output I need, it's all done within Aperture.
JT