According to Wikipeida:

Link bait is any content or feature within a website that somehow baits viewers to place links to it from other websites. Matt Cutts defines link bait as anything “interesting enough to catch people’s attention.”[3] Link bait can be an extremely powerful form of marketing as it is viral in nature.

Here’s a brilliant example - a funny parody of one of the most talked-about sites on the web. Creating it was not expensive, but thinking of it is pure gold. I’m linking to the source because they deserve it:

(yep, it’s too wide for this site - full image here)

(it’s English, so folk from other countries might not get all the jokes…)