According to the American Motion Pictures Association :

G : General audiences. All ages admitted.

PG : Parental guidance suggested. Some material may not be suitable for children.

PG 13 : Parents strongly cautioned. Some material may be inappropriate for children under 13.

R : Restricted. Under 17 requires accompanying by adults.

NC 17 : No one under 17 is admitted.

According to The Indian Censor Board :

U : Universal.

U/A : Children below 12 have to be accompanied by adults.

A : Restricts exhibition of the movie only to the adults.

S : For particular groups, e.g. doctors.