Linkjacking is a practice used to redirect one website's links to another. Usually, this is accomplished by submitting someone else's content to an aggregator website, which then drives traffic to the secondary site, rather than that of the original creator.
Some larger websites, such as Digg or Reddit, may linkjack a blog or an interesting article. In these instances, the notable website does provide backlinks and appropriate references to the original content, thereby gifting a large spike in traffic to the originating website.
From a negative standpoint, some linkjackers provide a website with no original content and by driving up traffic to the alternate content, they make money on any click through ads. When used in this fashion, the linkjacker doesn't provide backlinks.