Sharpening your images is essential in digital photography because of the way the demosaicing process softens images. It is often perceived to be a tricky technique to master because the controls involved can appear confusing and difficult to figure out. With a little knowledge though, you should be able to get the most from sharpening in Aperture.
There are three places in Aperture where you can sharpen your images. These all work a little differently, and will give you different results. The three are mentioned in the following:
RAW presharpening, in the RAW Fine Tuning controls
Sharpen adjustment
Edge sharpen adjustment
Of the three, Apple doesn't recommend you use the Sharpen adjustment. According to the Aperture documentation, Sharpen has been superseded by Edge Sharpen and is only provided for backwards compatibility. Edge Sharpen is a much more powerful and accurate sharpening tool, and it is wise to use this instead of Sharpen.