What do the different ratings mean?

G= General Audiences = All ages can read.
PG= Parental Guidance suggested = All ages can read with adult supervision.
PG-13= Parental Guidance under 13 = 13 yr olds and older can read.
R= Restricted = Only adults can read. May contain curse words or violent situations.
NC-17= No children under 17 = Contains adult situations and sexually explicit scenes. NOT for minors.

What is...

Yaoi = Male/Male gay relationship

Yuri = Female/Female lesbian relationship

Lemon = story with sex scenes between a Male and a Female