Too Long; Didn't Read
There are two main SEO types: Organic (natural) SEO, and non-organic (paid / artificial) SEO. The main advantages of organic SEO are that you don't have to pay for ads and at the same time you attract relevant users, i.e. users that are really trying to search for similar content, product, or services of yours. Non-organic SEO relies on paid search marketing approaches, such that of Google's pay-per-click (PPC) advertising solution.