Search Engine Optimisation (SEO) is a simple process of formatting your web pages for the search terms that your customers (or prospects) type in major search engines such as Google, Yahoo and Bing. If your website is not 'Optimised' for these 'Keywords' your website will not come close to being found on the first page of these search engines.
Optimising your website increases the chances of your website being found, therefore generating sales leads. Not all sales leads are genuine, not all enquiries you get from search engines will lead to a sale. However, the more targeted your keywords are, the more traffic/enquiries you will get, the better the conversion ratio you will get.
Targeted Keywords = good optimisation.
Making your website Search Engine Friendly is about developing the webpage structure in such a way, that any keywords that you optimise the webpage for, will display those keywords so that search engines will favour your webpages over non optimised webpages.