SEO is the bomb! It is one of my favourite traffic sources by far. There are so many things you can do to rank in Google and that means that there is basically unlimited traffic that you have the chance of getting! I have had a few, really big successful SEO websites, one generated me $7007 in a single day. The better you are at SEO, the more money you will make. Once you get really good, ranking anything in Google will be super easy because you know exactly what needs to be done.
SEO is not just used for CPA too. In our case it is, but there are so many online opportunities that having a site ranked in Google could be extremely helpful. I know guys who have a full time job as an SEO. THis stuff is powerful and this introduction will set you off on the right foot!