At its most basic, if you move your mouse over a Snap Shot enhanced link a preview of the destination site will appear in a small, non-intrusive pop-up window. If the link points to some sort of rich media (e.g. videos, images, audio, Amazon product shots, Flicker photostreams, stock charts, maps, etc.), users can view and interact with the media in question right then and there without leaving the original site.
Think of it as viewing through a window into the world behind the typical link on a site.
However, by including rich media support, this widget offers much more than a simple preview. Some might argue that in doing so, Snap Shots diminish the potential traffic gains of normal links to source content, but the user has everything to gain.
Runners up:
The rest, in alphabetical order:
