Link bait can basically be anything that people find interesting, but most often, its just a "interesting" article on your website.
There are many types of link bait - most link baiting is likely being done trough writing. But it can also be done by developing freeware software, blog and website templates, Graphic design. Etc.
Link baiting is a SEO marketing technique, most often used to promote a website or web page.
Link Bait Methods
A widely adopted link baiting method, is to create a tool that people can put on their website, but doing so they also place a backlink to the creators website.
Templates will usually contain a link back to the designers website, and thats while providing them alone can bring in backlinks.
Creating useful freeware tools can also prove to be a useful link bait strategy, but it can also be very time consuming, and it may require programming knowledge.
Other methods are generally dirty, and would often include lying about something, people would then link just to point out the lie.