Here are a few of the more significant differentiators between iPhoto and Aperture:
RAW handling: iPhoto imports and reads RAW images, but immediately converts to JPEG or 16-bit TIFFS for all edits. Aperture offers fully native RAW file processing throughout entire workflow, until you explicitly export the file in a specific format. The RAW Fine Tuning controls that are included (which let you customize the parameters of the RAW decode, to tweak contrast, noise reduction and sharpening) are not available in iPhoto.
Image processing: iPhoto creates a new file when adjustments are made. To create five different variations of an image means duping the file over and over. Aperture's adjustments are non-destructive; all adjustments can be removed, modified, and reapplied anywhere in the workflow. So you can create as many different versions of an image as you want, without ever having to duplicate the original file.
Metadata: iPhoto supports basic metadata, such as comments, title, film roll, and keywords. Aperture has full support for EXIF and IPTC metadata formats, with flexible display of metadata via tooltips, inspectors and overlays. Also, keywords can be organized hierarchically, instead of in a flat list, and can be attached to buttons for one-click application.
Multiple-image display: iPhoto can do 8-up; Aperture can go to 12-up, and can do simultaneous zooming and panning across multiple images (with iPhoto, you have to zoom and pan one image at a time).
Then, of course, there are all the features that are not available in iPhoto at all. The following are all unique to Aperture:
Stacks and auto-stacking, Light Table, Loupe, watermarking, color management/ICC profile support (for printing and export), multiple display support, Lift & Stamp (batch adjustments & metadata), monochrome channel mixer (for high-quality black-and-white conversions), Highlights and Shadows; black, white, and gray tint controls, white balance eyedropper, quarter-tone controls in Levels, color meter, Vaults etc...
I would say iPhoto is better for doing things like calendars, greeting cards, etc. Aperture doesn't have the templates for them. Aperture can do books, though and gives you much more control on the layout than iPhoto.
I have never played with Aperture: does it manipulate jpegs as well? At present I am usingphotoshop in a pretty basic way and picture by picture. In oter words: not very often. If Aperture allowed some clever workflow and let me tore the finished product in iPhoto that would be cool.
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I've been using Aperture for a month and love it. If you're looking to see what it does compared to iPhoto check out some of the videos on the Apple site, they'll give you a good idea. Versioning and non-destructive image processing in Aperture is beautiful - worth the price of admission alone. That said, one needs a powerful machine to run it. I'm backing-up my library right now and just about to install 1.5.