Trolls are a species of ugly and violent creatures who are often dim-witted, similar to how Ogres are portrayed in fiction and mythology. In in the Shrek universe, most trolls are depicted living under bridges(similar to the Billy Goats Gruff) and they appear slightly larger then Shrek(though smaller then many members of the Ogre resistance).
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.
- In the norweigan dub of the Shrek series, Shrek is called a troll due to the language lacking the word for Ogre. The troll that appears in Shrek Forever After is depicted as a Jotnar from Norse mythology instead.