Social bookmarking is a way of sharing your bookmarks with other like-minded people and also receiving the benefit of their useful bookmarks. It can, and has, been used as a staff development tool, a classroom/student resource and a way of promoting new ideas/technologies. The idea is to use client generated "tags" to organise the sites you have decided are useful. These tags are an informal way of categorizing all types of online content with keywords chosen by you.

The bookmarking tool I use most is Del.icio.us, which is probably the easiest to begin with.
Have a look at Us.ef.ul. It is a beginner's guide to Del.icio.us. The real power of Del.icio.us (and other social bookmarking tools) is the social network aspect. You can see how others have tagged similar links and discover other websites that may also be of interest to you. This allows an extensive knowledge network to be built and also be constantly upgrading.
Another bookmarking tool that can link to Delicious is Diigo. It offers an even greater range of options and even more learning network options