First, the difference would be more between (er/sie) frisst und (er/sie) isst (both irregular verbs). Usually, the difference is between people and animals. So, for people, you use essen - isst, and for animals fressen - frisst. But sometimes, if somebody eats very much, devours the food very quickly, so eats in a way that doesn't look very nice, then fressen might be used, meaning he/she eats like an animal.

http://www.linguee.de/deutsch-englisch/ is really helpful for things like these. Look up "essen" and "fressen", and from the range of translations you'll get a good idea about the connotations. (You'll also see some more unusual usages, as a beginner, you can ignore these.)