Essentially, these social bookmarking sites are search engines. But instead of a search engine algorithm out there crawling websites to find content and determine relevancy, you are bringing what you believe to be relevant content to these sites in the form of bookmarks. If others agree that your content (a bookmarked website) is relevant, that content scores more relevancy points.
Here's an example. I just searched for "social bookmarking" on Digg.com and found
a nice video that will probably tell you everything you need to know. This video (a bookmarked webpage) was in the second spot on Digg.com.
I did the same search on Google and could not find the same video.
What does this mean? Google's spiders don't view that video's webpage to be relevant enough to get on the first or second page, but 198 digg.com users liked (dug) that video, so it got top placement on Digg.com.
On the internet marketing and SEO side of things, these sites offer great article marketing, viral marketing and powerful backlink opportunities, but the content you bookmark better be novel or borderline outrageous if you want to have any chance of gaining a measurable benefit...
I hope that helps! Others, feel free to chime in if I've missed anything...