A Back Link is when another website adds a link aimed at your site, where one of their clients can click through and be taken to you. These can be hard to come by and they have a varying value according to Google. In the electronic mind of a search engine it sees a Back Link as a vote of confidence from one company to another. It also uses the amount of back Links you have to gauge how much confidence it should have in you or in Google speak what your Page Rank should be.
You need to get as many Back Links as you can, preferably from websites that a have a high Page Rank themselves. The more important the site the higher value the Back Link has.
Social bookmarking can very simply be explained as a method where internet users can store, organize and manage their bookmarks (sites of their interest) on a public bookmarking site instead of their computers. Not only that, they can also share their bookmarks with their friends or a network of people. Social bookmarking makes the task of saving sites for future reference much easier and more accessible.
Advantages of Social Bookmarking
The main advantage of bookmarking a site is the Increase in Traffic. Social Bookmarking sites are used by millions of people around the world and by bookmarking your sites/your blog posts with them, you automatically get access to the large volume of traffic coming to these sites. All you need to do is have your pages bookmarked with the various Social Bookmarking sites and if the content is good then people will surely save and share it with others. As your bookmark is tagged by others or as more people vote, its popularity rises and it moves to the front pages of the category or site. This leads to an increase in traffic and a higher Google Page Rank.