Off-page SEO, is doing seo outside your website..This includes linkbuilding..Blog commenting, article directory submission, guest blogging, webdirectory submission, creating press relaeses etc are example of Off page SEO..
Whatever you perform to market your site for driving traffic is called off page SEO. It includes lots of stuffs that you don't perform on your webpage but for your webpage.
Off page optimization is work from outside the owner’s website to improve the page rank of the website. Off pages works are increase the traffic and back link of your website. In according to my opinion there are included the work of off pages are:- Directory Submission , social bookmarking, forum posting, article submission etc.