An SEO Back-link is one that has a rel-follow tag and is clear to be and is from related content. It serves as a recommendation of your website or blog. Much like the link in this answer the URL not only points to a relevant blog post but the Text of the link can also contain search terms or keywords but keep in mind that with PANDA a range of links including natural links with no cover like this: http://thefreeguru.com/link-building-facebook/ and non related anchor text are also required to look like natural links.
Link building is still best done by dropping a few dollars into paid to post or press release and using article and website directories that allow you to submit a fair amount of related text because you can control the relationship and the syndication adds authority. We call it curated content now but it's still syndication and shared content.
Andy Zeus Anderson