Whats the difference between a NC 17 rating and a R rating?

Question

Whats the difference between a NC 17 rating and a R rating?

Answer 1

NC 17 is usually pornographic

Answer 2

NC-17 means that no one under 17 may attend EVEN with an adult and the movie can only be advertised at certain hours.

An R rating means that anyone under 17 can attend with an adult and can be advertised at anytime.  

Answer 3

R - Restricted.  Under 17 requires accompanying parent or adult guardian.

NC-17 - No one 17 or under admitted.