"Clickbait" can have a lot of definitions, depending on where you look and what your preconceived notion is of it to begin with. The definition that I felt best fit the bill was "an eye-catching link on a website that encourages people to read on." This can be in the form of, for example, an open-ended question, a controversial statement, or something bizarre and/or shocking.
The point of clickbait is to lure people into a website to drum up page views, not necessarily to provide "good" content. Although some legitimate news sources have subscribed to clickbait recently, its mostly used to give the reader a headline that doesn't quite describe the content of what they're about to click on, but gives them an incentive to go there just to check it out. Along with page views, sometimes websites use clickbait to bring readers to a page that requires some kind of payment or registration, or lead them to a page that is one in a series of pages they have to go through.
Many people view clickbait as a harmless distraction, but there is some ire in the internet community, especially when it comes to actual news sources using clickbait to grab readers' attention. In the age of information overload, not to mention the 24 hour news cycle, some people look at clickbait as just another example of sensationalism, rather than real news. However, others think that an eye-catching headline, even if it doesn't quite give the reader the most accurate depiction of the news story, is fair game–just a journalism trick of the trade to get people to look at their site for news.