5 stars are a very popular metaphor for visualising, very often sparkline-like, the popular opinion about an item. The values themselves usually come from voting, reviews and the like. The are popular because they show in a bar-graph like fashion what is the rating, and you can visually compare items. The downside is that as bar-graphs are quite crude, because they are one dimentional. It is not possible to judge if there is a consesus, or there are controversial opinions about the item in question. No idea about the distribution of data.
