If you mean social bookmarking sites here is a bad explanation: Social bookmarking sites are like user content generated directories that are always on the go. (May be inaccurate) For example: www.digg.com http://del.icio.us I hope that helps you understand a little
mmmm..... no, that didn't really help. I have heard that you need your links on the social bookmarking sites, but what is this kind of site, and how does a link on them help?
Here's a link with some brief explanations and a ton of social bookmarking sites on which you can list your links. http://3spots.blogspot.com/2006/01/all-social-that-can-bookmark.html
Here's a list of dofollow social bookmarking sites that I came across recently. http://www.caroline-middlebrook.com/blog/do-follow-social-bookmarking-sites/
Most of the popular bookmarking sites have nofollow on links. They are used mostly for traffic not to get backlinks. The article at wiki is pretty neat, have a look maybe you'll get a better image about how social bookmarking works http://en.wikipedia.org/wiki/Social_bookmarking
The vast majority of them use nofollow on the links which makes google ignore them. Social bookmarking can drive traffic to your site but it will not improve your PR, i agree with this fact social bookmarking is good way to bring huge amount of traffic & you can get few links for your sites.
Yes, but there are many dofollow using websites also...But these type of bookmarking are mainly targeted to bring visitors
Although social bookmarking sites will not give you any seo benefits, they are a great way to increase traffic to your site.