G
General (UK: Universal, suitable for all) - suitable even for very young children (age 4+).

PG
Parental guidance; mild swear words or light allusions to violence (age 7+).

PG-13
You should be 13 to read this, might contain some tempered violence and/or light allusions to sex (age 13+).

PG-15
Some darker or fairly violent scenes, allusions to sex and/or a homosexual relationship**, emotional turmoil, character death (age 15+).
**Homosexual Relationships are referred to as "slash". Don't like it, don't read.

R
Fairly graphic descriptions of violence, more detailed descriptions of sex/slash, strong or disturbing emotional unrest (age 17+).

NC-17
(classified "18" in the UK)
This rating is reserved for adult story-lines! Very graphic descriptions of sex/slash and/or strong violence. Unsuitable for anyone under 18!