The people from Google Webmasters launched a very good e-book called Search Engine Optimization Guide for webmasters. There’s explained what does Search Engine Optimization mean and what must a webmaster do to get a better place in the search engine.

The guide covers a lot of common areas like improving title and description meta tags, URL structure, site navigation, content creation, anchor text, and more areas that webmasters might consider optimizing. They are also giving you some advices for things to do or not to do on your website, some links to other resources that help expand the explanation of the topics in the guide.
They also say that they plan to update this guide at regular intervals with new suggestion regarding the optimizing of a website and more advices.

You can read more about this here and you can download the guide here (in .pdf format).