what is social bookmarking and how does it work for SEO ?

Since I have started learning SEO, I would want to know real time answers of what is social bookmarking and how does it work for SEO ?. To make it easy i would do bifurcation of my question into 6 parts as follows:
  1. What is social bookmarking ?
  2. What does social bookmarking site mean in SEO?
  3. What is social bookmarking used for ?
  4. How does social bookmarking work for SEO ?
  5. Why social bookmarking is important in SEO ?
  6. What value does it add to SEO ?
Answering your question "what is social bookmarking and how does it work for SEO" in simplest of possible language which will answer all of the questions attached to this question.

Social bookmarking is very important part of Search Engine optimization
It is the method used by web users to search, store, organize and manage bookmarks of the website pages on the net, that can used in future as per the requirement, this is done with the help of keywords and meta Tags
Some of very important Bookmarking sites are Digg , Del.icio.us Fark etc.
Bookmarking can be done in two forms
  1. public Bookmarking : In this case the bookmark will be visible to other users as well.
  2. private Bookmarking : In this case the bookmark is only visible to you.

This helps you in accessing bookmarks from any computer with online access.

If public, your bookmarks will be visible for the others, hence somewhere it is also called as social networking.
It has nows become a great tool for link building, promoting your business and showing up your brand presence in web

If you place your website links smartly you can get dofollow backlinks for free which will help build your search engine presence hence is part of Off Page optimization in SEO.

Taking SEO perspective Bookmarking is important since Search Engine's crawl them frequently, thereby helping your webpage to be crawled faster.

Social bookmarking site have huge visitor base hence you can even get unique visitors which is important for SEO.
inshort - Social bookmarking is a powerful tool for promoting a website and it is helpful in Instant Indexing i.e. Your website gets indexed by various Search Engines within quick time.