Rick Roll:

To send someone a link to 80s pop star Rick Astley's "Never Gonna Give You Up" video, making them believe that they're gonna see something completely different. The meme originated in 4chan's /v/ (videogame) board, when the official "Grand Theft Auto: San Andreas" trailer was coming out on the game's website but couldn't be watched due to heavy traffic. Playing a prank on everyone who was reading, an anonymous 4chaner said that the trailer was on YouTube and posted a link to Astley's song. Since then, the Rick Roll has been made countless times, not only on 4chan but on the whole of the Internet.

Source

Over the years there's been many forms of Rick Rolling on the internet, some fairly innocent like the example above and some downright nasty, which I refuse to entertain. I think the first and only time I was Rick Rolled was by the nasty variant. Believe me, you don't want to know, but some of you may already know, unfortunately. It took one time for me to learn my lesson. After that first disgusting example, I set the links to resolve in my browser's status bar when I mouse hover over the address and I haven't been Rick Rolled since.

Rick Rolling has become somewhat passé in modern terms, unless, of course, you're the US Government. There are a few examples of Governmental Rick Rolling below the fold.