Categories will list the facts in a visitor specified order, whether it's by popularity, age of the fact, truthfulness or controversy or, any other factors. Users may even decide to show facts in a random order. Furthermore, visitors may click on a random link to be taken to a random fact.
Categories can be anything that can't be defined by a tag. Such as "Alligators" can't be a category because they classify as an "Animal" and they'd be better used as a tag on the fact. This page will have a comprehensive list of all the available categories and maybe a random fact from the top 5 categories or something.
Displaying the Animals category:
Loaded in 0.03065 seconds.