Search Engine Optimisation (SEO) is the process of increasing the positions of a web site within the search engines, using careful analysis and research techniques.
A website needs to be written in a way that is considered to be friendly to Search Engines. If a website makes it easy for a website to be indexed (read) by the search engine spiders it has a greater chance of achieving good results in the Search Engine Result Pages (SERPs).
The first task of a search engine optimisation company is to understand how a search engine spider works, you have to know what the search engines are looking for and what techniques they give greater credibility to.