Page Importance PageRank (PR) is determined by the quantity and quality of links.
A link from another site is in Google view is a VOTE from another web site , therefore each link is a added vote which in turn will increase the PR value.
The more links, the more important google thinks your site is, it is also the quality of the linking web sites which has to be considered.
A link which has your targeted keyword listed on their web page as in the title will gain a greater importance and will therefore increase the quality of a link.
For more information about linking please click Website Design in Linking