SEO = Search Engine Optimization. This is a process of making a website more visible to search engines. This process can include a variety of things from Site Code, Site Lay Out, Site Maps, Robots Files, and other things that make it easier for the SE’s to crawl and index your site.
The process of SEO is something that is continually changing as SE’s continue to change and update the formula they use to produce results when people search for something. A site that is SEO’ed properly and uses as many of the techniques as can will rank higher in the Search Results than other sites.
Most SEO is done at the begining development and construction of the website. A good webmaster will know all the code that is necessary and that which is obsolete so that your site is seen well by the Search Engines. Parts of SEO also include CONTENT, which as I have stated is KING to a website. Well worded copy along with proper coding to meet the W3C standards for the web help with your sites SEO.
Other factors include the number of inbound links, the ability of site crawlers to index your pages, the use of various files and tools through the SE’s to help get your site indexed and reads properly to a SE Spider.