PG-13 is the rating where parents will decide it's probably best to stop taking their 5-year-olds to see this kind of film. PG-13 motion pictures can surpass PG-rated fare in terms of mature themes, sexual situations or sensual material, graphic violence without too much blood, but hella death, especially in action films, and limited use of harder language (ex: fuck, cunt, nigger, basically, any word a preteen COD player shouts over the microphone after they got sniped), but does not go so far as to deserve an R rating. The balding hacks over at the MPAA like to claim that the theme of the motion picture by itself will not result in a rating greater than PG-13, although Orgazmo got an NC-17 just for being a porn spoof. Depictions of characters using drugs will stay in PG-13 territory as long as it isn't outright substance abuse. Brief/partial nudity is also allowed, and yes, you can show tits. Sexually-oriented nudity is still considered to be a no-no, on the other hand. If a motion picture uses a harsh sexually-derived swear word (such as fuck), but without the sexual meaning, its lowest rating prospect is PG-13. A big shiny R is deserving according to the MPAA if the film uses it more than four times or once in a sexual content.
Nonetheless, exceptions to the rule do exist. There are a few PG-13-rated films where the F-word is used multiple times. Just look at The Hip Hop Project and Gunner Palace; the latter uses it 42 times, two of those sexually. (To be fair, it was originally rated R...) Several intense scenes of violence, language, or sex are cut out of potentially R-rated films so that filmmakers can target a specific audience. The most common films to be rated PG-13 are summer blockbusters involving superheroes or well-known action stars, or documentaries that don't contain too much profanity or violent depictions. Generally speaking, this is the MPAA's highest unrestricted rating, meaning kids under 13 still have full access to the film. That doesn't stop parents from cracking down on that shit, and unfortunately, nobody's stopping them.
PG-13 means you can bash in a person's skull, but don't make it too bloody, alright?