There are two aspect ratios that matter:
The camera aspect ratio
The output aspect ratio
Compact cameras and 4/3rds DSLRs (Typically Olympus and Panasonic) usually take pictures in a 3 by 4 aspect ratio. This ratio is ideal if you are going to print at 8x10, 8.5x11, or 11x14. Only a little cropping of the long end is necessary. The problem is when you want to print 4x6 prints. You will need to crop a pretty good chunk off the top or bottom (or both).
Other DSLRs, as well as 35mm film cameras, use a 2 by 3 aspect ratio. This is perfect for printing 4X6 prints without having to crop. But if you want to print at 8x10 or 11x14, you are going to really have to whack the long end to make it fit.
For the web it’s usually no big deal. A common size is 800 pixels by 600 pixels. This is native for a compact camera but a 2x3 is 800 by 533.