Off-page SEO refers to all actions undertaken outside, but also inside a website, to establish and improve its ranking on search engine result pages (SERPs) by developping netlinking.
Off page SEO refers essentially to link building (in fact backlinks building). With Google fighting more and more inorganic backlinks, off page SEO is increasingly the search of quality link building.
Off page SEO is a misleading term because some of the actions are actually undertaken on the website pages to gain backlinks outside. But the good signs (backlinks) of popularity and quality content which are taken into account by Google and others are off the pages.
Of course, do follow links are especially sought after for off-page SEO.
Techniques used for off page SEO are essentially:
quality content development for gaining natural backlinks
social media optimization
online press releases
Many black hat SEO techniques are used for off page SEO:
content spinning (for creating website networks)
Off-page SEO is complementary to on-page SEO and the relative importance attributed to these two aspects varies from case to case.
See also SEO.