SEO stands for Search Engine Optimization.
It focuses on achieving the highest possible ranking in a search engine such as Google. This is because the higher the position of a site in a search term is, the more people are likely to visit the site.
Lets take, for example, the term; Webmaster.
If
www.webmasterserve.com was the first result on Google for 'webmaster' it would recieve a large percentage on the people searching for that term on Google. However, if it was number 432 it would recieve almost none of that traffic.
Therefore, the main aim of SEO is to push your site up the search engine listings so you recieve a larger share of the market.
The easiest way to do this is through using a publishing program such as Google Adwords. Adwords allows you to pay to have your site in the highest positions of a search engine and therefore will allow you to pull in the most traffic possible. However, this is an expensive option, especially with some of the more expensive search terms. You could be looking at multiple dollars for everytime someones visits your site, which especially for smaller companies or sites, is not really worth it.
However, certain search terms are considered less expensive and smaller companies and sites can opt for them instead. While they do not get as many click throughs, they will still get traffic. Essentially, you get what you pay for.
There are ways however of not having to pay for high listings on search engines and this is where SEO comes into play.
Search engines use 'crawlers' to crawl sites looking to add them to search engines. They look specifically for two things.
The first thing to do is to make sure your content and site is 'keyword rich'; in other words it contains lots of words that can be indexed by search engines.
Secondly, you can also use 'meta tags' which can be added to each page, or the site as a whole. These add keywords to your site which the search engines can 'read'.
Excessive keywords and links on sites can lead to blacklisting from search engines. This is where the search engines will no longer crawl your site as it sees your site as being not organic. It is always important to make sure your site looks organic and that search engines do not class your site as not being so, as it is a pain to get your site off a search engine black list.