What is Link Juice?
In order to rank well in the search engines, web sites need links to them from other web sites. Generally the more links to a web site, the higher it will rank in the search engines. There are a lot of other factors that go into ranking a site, so it is not strictly true that the site with the most links will rank the highest. Not all links are created equal. Link juice is the term that has been coined to describe how much benefit a link can pass on to the site the link is pointing at in terms of helping its ranking.
The links that pass the most link juice are those links that are editorially given and not got through self-promotional methods (eg forum signature links; blog commenting; directory submission); those links that do not have the nofollow attribute added pass link juice; the links that are on pages that have a high Google PR pass more link juice; links on pages with a small number of links pass more link juice; links from high ranking, high authority and more trusted web sites pass more link juice. These are the sorts of links that the search engines like to give good weight to when deciding how to rank sites.
The links that pass no link juice or minimal links juice are those links on low Google PR pages; have lots of other links on the page; is not relevant to the topic of the site that it links to; is gained by self promotional means; and is from a domain that is not as trusted much.