No onscreen sex or nudity, just tons of profanity and sex talk. It's not that bad but definitely not for anyone under 14, even then it may be innapropriate.
Clerks is our first introduction to writer/director/actor Kevin Smith's "View Askewniverse". It's also the greatest film in that series. The characters of Dante and Randall engage in interesting, intelligent, and hilarious dialog while dealing with the boredom, hypocrisy and occasional weirdness of everyday life in a convenience store.