{G} General Audiences. G-rated stories are the kind anyone can read. They have no sexual innuendos, no swearing, no violence, no nudity, no nothing. You can remember this one by associating G with the word Good.
{PG} Parental Guidance Suggested. This means there may be something too scary for a kid or something they may want explained to them. Commonly found in PG fics are: very mild swearing (one or two words), mild thematic situations, cartoonish violence, and very mild sexual references (most kids won't even pick up on these).
{PG-13} Parental Guidance Suggested for Children 13 and Under. It's amazing the huge jump between PG and PG-13. PG-13 fanfics are likely to have: swearing (though the "F word" is limited to a one or two-time use), violence, sexual references, "a scene of nudity" (usually only detailing the ass, side of the breasts, or the form of a human body), drug references, and thematic elements.
{R} Restricted. Here's where you need to start exercising caution and remembering what you can and cannot handle. R-rated fics may have: heavy swearing (including consistent use of the "F word"), bloody and gross violence, strong sexual references and blatant sexual speech, nudity (describing the breasts or human body in a bit more detail than PG-13), drug use, and thematic elements.
{NC-17} I don't remember exactly what that stands for, but if something is NC-17 you cannot read it unless you are 18 or older. By clicking on a link to an NC-17 story you are saying that you are 18 and if you're not 18 you're the only one to blame if the effects of the story cause you mental, phsychological, or emotional damage. NC-17 fanfics will describe sex vividly between a man and a woman, a man and a man, or a woman and a woman; and give detailed descriptions of sexual things (contraceptives, sexual terms, private body parts, etc). They also have a lot of R-rated things in them as well.