Linkjacking means different things to different people. Many see it as using the content on one site as the bait to get viewers to pass through your site, or even stay there and explore without going on to the primary content. Most of the time, there will be tidbits or summary information about the primary story accompanied by an interesting image or bit of photoshop magic that takes up most of the page.
To many, including Urban Dictionary, a true “linkjacking” requires that a person from the website doing the linkjacking also submits the story to an aggregator such as Reddit or Digg to drive traffic. I believe that websites and blogs that have a strong enough following to be able to “assume” someone will submit the story are also linkjacking, even if they are not the one’s doing the submitting.
The idea is to generate traffic from social media sites and even the search engines without having to write a ton of original content or do the research. Here is an example of a website that I like a lot, Engadget, which is notorious for linkjacking: