As far as my knowledge in this field is Off Page SEO and Off Site SEO is same. All the activities which is performed on website rather than the main website is known as OFF PAGE SEO. People used to call it Off site SEO because of the activities are performed on OFF site instead of Main website.  Off page seo helps in building quality backlinks and driving traffic to main website through the activities.

Ex : If you are using social media and writes something on any niche or topic and include a website link the relevant traffic who are reading your post is likely to click that link and redirects to the page to know more about. Same happens with other off page techniques.

Some of the off page activities is mentioned below:
1. Social Bookmarking
2. Web 2.0
3. Blog
4. Article Share
5. PR share
6. PPT share
7. Image Share
8. Audio Share
9. Guest Blogging
10. Classified submissions, and the list is so on.

Regards!
Platerzone