A backlink is a very simple thing. All a backlink is is a link on another site that leads to somewhere else.
So a backlink to Facebook on Hubpages.com looks exactly like THIS:
A Backlink to Hubpages.com on Facebooks looks just like what you'd see from anyone hitting the "Like" think for Facebook up above here.
A backlink merely suggest to someone," hey, look at this web page here!"
Some backlinks have some value that others do not have. Look at Facebook - a backlink to YOUR hubs there is a very good idea. It's a wonderful idea because there are TONS of people on Facebook at all hours of the day. A backlink on Myspace is not nearly as good an idea as is a backlink on Facebook.
Guess what? I still put backlinks on Myspace too because internet web spiders still index links on that site - so while backlinks are really good things to spend some time creating - not all of them have the same value.