We cannot explain it better than MarketerGizmo.com.
Google makes changes to their algorithm at least monthly, if not weekly. And with every major update at least, search engine optimization seems to get a little more complex.
Back in the early days, on page SEO was basically synonymous with keyword stuffing, while off page SEO involved little more than getting as many links as possible as quickly as possible to your pages.
Fortunately for everyone who has to use a search engine, both on and off page SEO have evolved significantly. Each component of search engine optimization now has its own components to attend to, and each one offers its own potential for ROI.
This overview will walk you through the important distinctions between on page and off page search engine optimization so that you can prioritize your SEO tasks effectively.
Read more HERE.
And if you want to spend a few hours a day and do this yourself, here’s a handy guide: http://www.marketergizmo.com/comprehensive-guide-to-on-page-and-off-page-seo-techniques/