In SEO, we have to know two things are very important. The first one is open directories project (ODP), which is dmoz.com. The idea of open directories is to submit your site to other big web hosts' directories in order to increase the listed frequency from the searching results by search engine. The second important concept is crawler-based system. Crawler sometimes is also called spider or robot, which is a smart collecting program from search engine to gather many information from web sites in the Internet. After collecting, crawler (spider or robot) will return to search engine and update the new information to the index of search engine's database. Then, search engine's searching algorithm can find out your site in its indexed database based on certain keywords.
Extra concept of improving SEO can be "link", linking farms, whcih is to make as many friend-links to your site as possible to convince search engine (or crawler programs) that your site is highly associated with other sites. Then, it is possible to increase the listed frequency of your site. Another concept such as cloaking, which is the technique of making the deference of looking to human and crawlers. It is usually used on Flash-made web pages and doorway pages.