When it comes to SEO, one of the major factors in getting a higher Google PR is inbound links.

Inbound links from any site used to be enough.  Not any more!

Due to link farms and endless inbound links from sites that have nothing to do with your site, not all inbound links are good.

Now, more than ever before, it is important to choose where your inbound links are coming from. 

The links should come from sites that have something to do with the type of content you have.  For example if you have a website about dogs, links from a used car website would not be good.  Instead you want links from say dog food companies, veterinarians, and other pet related websites.

Another important factor in how much that inbound link will help you is the number of other links on that page going to other websites.  The fewer links the better!

If you are soliciting getting your link placed on a site look at the page you are prospecting and its Google PR.  The higher the referring pages PR is the better that link will be for you.

In the end search engines, like Google, are getting smarter and smarter and instead of risking possible blacklisting, do not join the link farms and be very selective who you ask to display a link to your site.