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
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.