Clickjacking is the hijacking of your click, unbeknownst to you. A victim may not even know that the click has been redirected, which means there could be clickjacking attacks going on that no one knows about yet.
The technique was used in a series of prank attacks launched on Twitter. In that case, users clicked on links next to tweets that said "Don't Click" and then clicked on a button that said "Don't Click" on a separate Web page. That second click distributed the original tweet to all of the Twitter user's followers, thus propagating itself rather quickly.
Clickjacking attacks are accomplished by creating something called an iFrame that allows a browser window to be split into segments so that different items can be shown on each. This code is inserted into the target Web page and is invisible to the end user. When the end user's cursor clicks on the section of the page where the malicious iFrame is hiding, the attack is launched to do whatever the attacker desires.
An attacker could hide an iFrame under any innocent link on any Web page--a headline on The New York Times or a "digg this" button on Digg, for instance--and when the victim clicks on the link, the cursor is actually clicking on the hidden iFrame.