How SEO Works ?

In order to understand Search Engine Optimization, one must first understand how search engines work. Actually, the term “search engine” refers to two types of engines: crawler-based and human-powered. A crawler-based search engine sends what is called a spider to go through web sites. The spider's findings are then added to the engine's index. The spider returns to the web site periodically and brings updated information back to the index. When a search is conducted, the engine sorts through the entire index looking for matches. Google is one example of this type of engine.

Human-powered directories require a short description (written by either their reviewers or the company running the site) that is included in the directory. Updating pages on the site does not alter the listing and searches only look through those descriptions for matches. Look Smart is an example of a human-powered directory. Today, many search engines, such as Yahoo and MSN, present both crawler-based and human-powered results. These are known as hybrid search engines.

When people discuss site ranking, they are generally referring to the crawler-based search engines, which list search results based on relevancy. But how is that relevancy determined?

Search engines determine relevancy by applying a set of rules known as an algorithm.
Which each engine's algorithms are different, they all look for similar things:

Location of Keywords – Search engines first check title tags, headlines, and the first two paragraphs of content for keyword matches

Frequency of Keywords – Search engines look at how often a word appears on a page. The more frequently the word shows up, the more relevant the site.

Avoid Spamming – These are elements that cannot be controlled by SEO, such as link analysis and click through measurement.