mpaa film ratings:

• g: general audiences. all ages admitted. nothing that would offend parents for viewing by children.
• pg: parental guidance suggested. some material may not be suitable for children. parents urged to give "parental guidance". may contain some material parents might not like for their young children.
• pg-13: parents strongly cautioned. some material may be inappropriate for children under 13. parents are urged to be cautious. some material may be inappropriate for pre-teenagers.
• r: restricted. under 17 requires accompanying parent or adult guardian. contains some adult material. parents are urged to learn more about the film before taking their young children with them.
• nc-17: adults only.

ao3:

• general audiences: the content is unlikely to be disturbing to anyone, and is suitable for all ages.
• teen and up audiences: the content may be inappropriate for audiences under 13.
• mature: the content contains adult themes (sex, violence, etc) that aren't as graphic as explicit-rated content.
• explicit: the content contains explicit adult themes, such as porn, graphic violence, etc.