First of all, Marvel withdrew from the Comics Code Authority (that we all grew up seeing on the front cover of comics), in 2001 and established its own system when the CCA refused approval of a comic book, X-Force #116 (click the link for the Marvel Unlimited issue).
Marvel implemented its own system, but the page hosted at Marvel.com no longer exists. The only details about it are on Wikipedia here:
For your convenience, I’ve copied the text from the Wikipedia entry:
|The Marvel Rating System assigns each comic book one of the following ratings:|
The first Marvel Rating System, implemented in 2001, used the following categories:
Until next time,