People often mix up the terms SEO and SEM. They are actually two entirely different things. so I will clarify both terms below. We are basically talking about organic search engine optimisation (SEO) versus paid search engine marketing (SEM/PPC).
SEO = Search Engine Optimisation
When you do SEO for your website, you optimise it for the search terms (SEO keywords) you want to rank for in the search engines, so that people will be able to find your site better in the ‘normal’ (organic) search results.
If you know how to optimise your website and do it yourself, SEO doesn’t need to cost any money, only time and knowledge are needed.