G- General Audiences. These stories have abosolutely nothing offensive in any way shape or form. You will never see "G" on this site, unfortunately.
PG- Parental Guidance suggested. These stories may be unsuitable for children under the age of five. In other words, there is very little offensive about these. At most, a teeny tiny bit of romance or action.
PG-13- Parental Guidance suggested under the age of 13. These stories will contain romance, some language, maybe a little bit of "snogging" that puts them at a higher rating than most. Also, this is when the angsting and violence starts. Everything under "X/1999", just by default, falls into this rating or higher.
R- Restricted.Under 17 will not be admitted without a parent or legal guardian over the age of 21-- according to the American film assosiation. These stories will contain violence, limey sex (in other words, implied or partially viewable), heavy angst, blood, death, destruction, and all that other fun stuff.
NC-17- No admitance under 17. Literally just short of pornography. This is a misconception by most people, but porn is rated "X", and that means you must be 18 or older to so much as touch it. This... well, lets just say that I trust you are either old enough or mature enough.You will not be warned again.