Search engine optimization (or s.e.o. as it is commonly referred to) is split into two different types, paid S.E.O. and organic S.E.O.
Organic S.E.O. is the most valuable form, as this is about the optimization of the content of your website and not simply paying to appear on the first page of results for your desired terms.
The process of reviewing content on your website includes the content on the pages, links and meta tags. Often, the result of search engine optimization is an increase in visitors to the website as your website is appearing in more searches by potential visitors.
There are a number of factors that come into a good website, including design, user friendliness and of course search engine visibility. All are as important as each other and without all three, your website simply will not see the level of traffic you would be desiring as your site won't be found in searches or people will not stay on the site because they are unable to find the menu easily or simply cannot find what they are looking for.
This can even go the other way whereby a site is created that while pleasing to the eye of the consumer and easy to follow, is overly complex and cannot be read by search engine robots, therefore resulting in minimal content being indexed and the site not appearing in search results.
This is of course a very basic summary of S.E.O. however if you take these principles and apply them to a review of your website, you will be on your way to a search engine optimsed website.