Sometimes it seems like all of life has the same themes as high school: what's important is being popular. A significant measure of popularity on the Web is how many inbound linkslinks from other sites to your siteyou have.
Inbound links are an important component of Google's PageRank systemwhich is a way to order the sites returned from a search.
Obtaining inbound links is not rocket science, but it is labor intensive and does require some thought. The best way to get another site to link to your site is to ask for it, as obvious as that may seem.
Link farmssites that exist for the sole purpose of providing inbound links to better a page's search rankingwill not help your site become more popular, and are even likely to damage your standing with Google and other search engines if they are noticed.
It makes sense for sites to link to your site when they have similar or related content. This is a reasonable thing for a webmaster in charge of the other site to do because it adds value for his or her site's visitors. (If your site is not adding value, you might want to rethink its premise.)