writing and design of your blog or website is onpage seo. Getting links to your website from other sites is offpage seo.
In simple classification, on page optimization is something in which web page content, images, coding, style sheet are modified, to make them search engine friendly. While in off page optimization, we don't make any changes on the site, but we try to increase our domain presence on internet, try to publish information, try to reach targeted audience, basically try to be at every place where our users can be, or where our products or services can be sold. These activities are done in the off page optimization.
Well, on-page optimization is the work that we have to work on the website directly while the purpose of off-page optimization is to get back links toward the website so the popularity of the website will increase... there are too many activities that you can perform in the off-page optimization like as follows: Forum posting Blog commenting Directory Submission Press release etc