According to Wikipedia, Search engine optimization is a set of methods aimed at improving the ranking of a website in search engine listings.
search engines developed more complex ranking algorithms, taking into account factors that were more diverse, including:
- Text within the title tag
- Domain name
- URL directories and file names
- HTML tags: headings, bold and emphasized text
- Keyword density
- Keyword proximity
- Alt attributes for images
- Text within NOFRAMES tags
Every search engine company have their own algorithm rank the website that crawled by their engine. Usually people consider Google search result as parameter on SEO. You can ask Google consultant to help SEO for your website :D
More resources:
http://en.wikipedia.org/wiki/SEO
http://www.seochat.com/