PG-13 originates from teh american movie industry and sets a rating for movies suitable for children. See
http://en.wikipedia.org/wiki/PG13#Th...e_PG-13_rating
On a website this indicates that the site should be suitable for minors under the age of 13 with Parental Guidence and over 13 without guidence. To implement such a rating we don't allow any nudity or violence on our forums.