The MPAA (Motion Picture Association of America) you'll recognize from the green background on US trailers. [see Wiki]
The BBFC are the UK censor ... but they dropped the word censorship from their name quite smartly to take on a more neutral name ... the British Board of Film Classification. Their website is excellent, including a student-specific section, and a quick search tool to find their rationale for specific movie ratings.
In most cases, the 18/NC-17 rating is box office cyanide - see this post on Zombie, and here, where I analyse the record of 18-ratings including Fifty Shades of Grey, an exception. (There's also the possible F for feminist rating, the Bechdel Test applied in Sweden!). Wal-Mart won't stock any NC-17 DVDs, a serious consideration as the biggest DVD retailer in the US!
I include the BBFC explanation for 3 case study films in this post: This is England, Hot Fuzz, The World's End.
Do the regulators favour the studios and treat Indies more harshly? There's certainly a case to be made, and the 18 ratings of TisEng and Sweet Sixteen seem to be quite typical, as are the 12 ratings of films like The Dark Knight. You can make an interesting comparison of Warp and Working Title on this.
The following trailer is the ironic, entertaining (NB: and UK 18-rated) documentary investigating the ultra-secretive MPAA, This Film Is Not Yet Rated (IMDB; Wiki). In the full film you'll hear Trey Parker comparing his poor treatment by the MPAA when he submitted his Indie debut feature ... and how this changed utterly when he was behind the Sony-backed South Park movie.
(Can't embed the trailer: here's the link)
Here are the two ratings systems graphically compared.
|(CMD/CTRL-) Click picture to enlarge|