Link building strategies have, for most people for a long time, revolved around reciprocal link exchanges. Whilst most people understand that links are important, they generally don't understand why this is so. In a nutshell, a link to your site has traditionally been accepted by Search Engines as a vote for your site. A link from a topic or theme-related site to yours is better than a link from a site having a completely different topic. An important site's link to yours carries more weight - for example from The Open Directory, or Yahoo Directory. All pretty straightforward...