Search Engine optimization
Search engine optimization (SEO) is the process of improving the volume and quality of traffic to a web site from search engines via "natural" ("organic" or "algorithmic") search results.*
Optimizing your web site for the major Search Engines is the single most cost-effective way to increase your web site’s traffic, generate leads, and obtain additional sales with-out paying for advertising.
When done correctly, SEO only needs to be done once and then monitored and tweaked as needed. Succeeding on the web is not that difficult, but to do so, you need to be where they are looking. There are 150 million searches done on the web every day. If you're not appearing in the search results, then It’s time you made your site more visible.
What is SEO
http://www.google.com/support/webmasters/bin/answer.py?answer=35291
Footnotes:
* Description taken from Wikipedia