A common measure of website relevance and usefulness is there link popularity. Achieving a very high popularity is the ultimate indication your site is highly regarded. It's a measure of how your site rates alongside the thousands of other sites in the same marketplace.

A high number of quality links from quality websites will increase it over time, while poorly worded links from irrelevant sites with low PR, will have substantially less effect.

Webmasters often seek to increase there link pop, by using link farms, or software programs designed to automate the process. The most blatant and poorly implemented of these software solutions encourages linking to almost any site, with no real optimization or effort to ensure the link is a good one.

Other solutions provide you with a mile of email addresses and a pre-written email template you can use to ask other webmasters for inclusion on there site. Neither of these methods is terribly effective, and since this method is often seen as spam, it may also get you banned from the SERPs.

There is a growing trend towards buying and selling links. This can be an excellant way to earn extra revenue if your site is popular, or gain a leg up on the competition, by buying several links from "authority" sites in your chosen sector. SEO professionals suggest this method can be genuinely useful, while the major search engines warn against this practice and advise caution.

If you choose to buy links, make sure the site is reputable, make sure you get full disclosure on where the link will be placed, make sure the link is keyword targetted, clickable, and does not use the NOFOLLOW tag.