'Backlinks' are one of the major influences on Google when it comes to ranking your website, but it is important to realise that not all backlinks are equal.

Many site owners focus on the quantity of backlinks they can generate, but it is quality of the link that is important. A website with a handful of good-quality backlinks can rank higher than a similar site with hundreds, if not thousands, of poor quality ones.

How do I know if a link is good or bad?

The Nofollow Rule

The first thing you must do when considering whether to request a link from a site is to look at how they manage their existing links. If you right click on a page in your browser and choose 'View Source' or 'View Page Source' it will open up a window showing the raw code for the page. Do a search and look for the "<a href" tag, which signifies the start of a link on the page. If it contains the code "nofollow" then having a link back to your site from this is a waste of time. The link will be worthless in Google's eye and will not add anything to your search engine ranking.

The is most important rule: only request links from websites that do not use the 'nofollow' attribute.

PageRank

Look at the PageRank of the site. The higher the PageRank, the more importance Google will place on the link, and the more value it will add to your own ranking score.

If you have the GoogleToolbar installed on your browser it will automatically show you the PageRank of the particular page you are viewing. Alternatively you can use the free resource at http://www.prchecker.info/check_page_rank.php.

However, don't ignore sites with low or no PageRank. As long as they do not use the 'nofollow' attribute they will still have some value.

Keyword Relevant Pages

Search engines will give more importance to links that are from pages/websites relevant to your own content.

For example, a link to your website using the term "it support" will gain a better score if coming from a page entitled "How to Choose an IT Support Provider?" over a page entitled "Businesses in London".

This applies to links from Blogs, Articles and Press Releases, which you can create yourself!
 
Sleepy Frog Design's Website Healthcheck not only tells you what keywords you should be targeting but can also help you locate the most valuable backlink opportunities.