Is it based on user ratings or user lists or a combination of the two? Thanks.

Text taken from http://rateyourmusic.com/faq/#q219772

"How do the top 25 and top 100 lists work?

Many factors are used to calculate the Overall Album Rankings. It is a complex algorithm involving album rankings, user participation, album popularity, etc. There are also built-in safeguards to protect against sabotage. The Album Rankings are updated about once per week, as the calculations use a tremendous amount of server resources. The ranking algorithm is constantly tweaked and modified, so what's top-ranked one week may change the next."

Thanks.

Those safeguards seemingly aren't good enough to protect acts with more fanatical fans than most, such as Elvis Presley, Rush, Michael Jackson, Kiss, etc. against overperforming in the lists though :)