Matt Cutts, the head of Google’s Webspam team, defines link bait as anything “interesting enough to catch people’s attention”.  And it’s a good thing that can increase your ranking on Google.

Link bait can be any content or feature in your website that will attract attention or persuades others to link to your site. Because of its viral nature link bait can be an extremely effective form of marketing and it can be implemented on a very low budget. From an SEO perspective having high quality inbound links helps, it is one of the many metrics used by Google and other search engine ranking algorithms to determine the ranking hierarchy and good link bait can increase the number of these links to your site.