There are many ways, some traditional and some quite innovative, to get other web sites to link to yours. That said,, it can be easier said than done. Google defines a link as it pertains to rankings and SEO as a "vote" from one site to another. The more quality votes your site receives, the greater chance you have of rankings well. If a well established site links to yours, that link carries more weight than one would from a  less reputable page.

If your site has useful content and is doing something unique, you're already ahead of much of the competition. People need a reason to link to your site, as very few will do it out of the goodness of their heart. Trading links can work, but link exchange networks have decreased in value and won't be of much use in competitive fields. Buying Text Links, if you haven't heard, is a big Google no-no. While entire articles could be written on this topic, here are a few popular methods of acquiring incoming links:

1)     Create and issuing press releases with a link back to your site

2)     Submitting to reputable business directories such as DMOZ

3)     Start participating on related blogs by commenting and exchanging ideas

4)     Ask clients with web sites, if they would add your link to their "partners" section

5)     Participate in relevant forums and discussion boards with a link in your signature

6)     Get involved in social media and bookmarking websites

Regards,

SEO Blogmaster Matt