What is SEO (Search Engine Optimization)?
Search engine optimization (or SEO) is taking a website that is built by humans and making it easy for both humans and search engines to digest what the content of the website is about.
Why is SEO necessary?
As brilliant as search engines and their algorithms are, they are still not all seeing or all knowing. There are limitations to what the spiders can find and index.
If you have poor linking structure, then search engines may not reach all of the content that is on your site. Or, if certain content is not prominently linked to, then it may be demeaned unimportant in the search engine indexing of your site.
If your site uses a lot of flash, video, audio, and images for its content, then search engines will not be able to clearly interpret it and will need some explanation of what your site is about. Also, all text that is not in standard HTML format will be invisible to search engines.
You can read more about how to optimize your site and how search engine work in these related articles.