In the USA the MPAA rates movies according to the appropriate audiences on a scale of G, PG, PG-13, R or NC-17. Only the last two impose any constraint on who can see it (for R an adult must accompany the viewer, for NC-17 only people 18 or older can see it).
In the UK the ratings are U, PG, 12-A, 15, 18 and R18. The numeric ratings mean nobody under that age can see the movie (except for 12-A where an adult must accompany under 12s but they really shouldn't be shown to under 12s according to the advice; R-18 is reserved for, basically, porn shown in specially licensed premises). Other European systems are usually similar to the UK but with different details.
Many Europeans are mystified at the American system which seems very tolerant of violence but puritanical about anything sexual. They are also mystified that the American system's lax attitude to age restriction leaves a whole category (NC-17) which many cinemas won't show when the European mandatory age restriction avoids this by making the rating clearer so audiences can safely choose appropriate content.
But the criteria for putting movies in different categories also seem to be very different. What are the key differences between the movie content that generates the different categories in US-style and European-style ratings systems?