On Page is all the actions that you have done on the page / site that you want to promote, and off page contain to external activities (for example: link building).
As part of this service, you have to adjust all your site pages to the phrases that selected to promote.
A lot of people from the SEO area claims that On Page has no significant impact when it comes to competitive phrases.
Although many argue that a relatively small benefit, (I personally know many cases where sites progressed really well when they put emphasis only on the On Page elements). I always invest in On Page from two reasons: It helps me with long tail phrases (something that if I wanted to do only through the links, I'll have many more resources), and it also contributes to the users.
If you do On Page by yourself, do not involve any costs related here but time. It is important to learn how to do it right so as not to create damage. My recommendation is to do it alone because the knowledge that you gained from learning, will help you to make important decisions in more advanced stages of promoting the site.
--------- edit by SEO Bunny -----------
Changed Title from "On Page" to "Some OnPage Experience"