Trolls are a species of ugly, dim-witted, violent creatures, similar to how Ogres are portrayed in fiction and mythology. However, unlike ogres in the Shrek universe, they are still portrayed as a dumb species. It was also mentioned in a YouTube video starring Shrek that trolls are infamous for living under bridges and writing mean comments to artists. They even write in all caps with terrible spelling, and get snot all over their papers. Luckily they are defeated if you don't "feed them" with reactions. In other words; they are the medieval equivalent to internet trolling.