For the ease of users, any ratings system can be used, provided it denotes content of the fic correctly.
To cover things, here are the British and American film classifications, which many people use when it comes to rating fics, because they are easily recognised terms. Obviously it should be noted that the highly acclaimed BBFC and MPAA do not endorse this, and are not affiliated with the comm.
GB / USA
U / G
PG / PG
12 / PG-13
15 / R
18 / NC-17
U - Suitable for all, but parents are advised that certain scenes may be unsuitable for children under 3 years old
G - All ages admitted
No nudity, no sex, and no drugs, violence must be minimal, and there may be language that goes beyond polite conversations.
PG - All ages admitted, but parents are advised that certain scenes may be unsuitable for children under 7
PG - Some material may be inappropriate for younger children.
There maybe some profanity and brief nudity, mild violence, or very mild drug references.
12 - Suitable for those aged 12 and over.
PG-13 - Some material may be inappropriate for children under 13 years old.
May contain very strong language, nudity (non-explicit), strong, mildly, bloody violence or mild drug content.
15 - Suitable for those aged 15 and over.
R - No persons under 17 without parent or adult guardian admitted.
May contain very strong/sexual language, mild explicit nudity, strong/gore violence or strong drug content
18 - Suitable for those aged 18 and over.
NC-17 - No One 17 or Under Admitted.
May contain very strong/sexual/offensive language, strong explicit nudity, very strong/gore/disturbing violence, or strong drug abuse.
Writers should be aware that these are not final, and to use their own discretion. As a rule of thumb, it is better to err on the side of caution rather than rate too lightly.
Any queries, or potential additions to this post, please comment and I shall amend as appropriate.