Whats the difference between parental advisory, 18+ and strong impact course language?

ive got a few rap/hip-hop cds, and i got a few that say parental advisory (eminem), some that say strong impact course language (eminem, T.I), and a few that say 18+ (tupac, D12)....just wondering what the difference is. im in australia, i know america mainly uses parental advisory, but is there any difference between them? and with the 18+ ones, they are usually really strict depending on where you buy them from, im only 16 but most of them i got from itunes anyway, but yeah.....whats the difference???