Link bait is publishing great contents on your site that will generate lots of links from people who find your content helpful. Those links tend to be quality links and they are strongly recommended by Google.
Link bait is also a blogging promotional strategy where bloggers post something very controversial or they start a 'blog war' with another blogger. In the process the controversial material gets links simply due to other bloggers criticizing their extreme controversial views or the 'blog war' generates links from other bloggers who are mentioning the big disagreement / fight between two more prominent bloggers. 'Blog wars' often turn out to not be real fights at all but just done to pull links in to both blogger sites.