What is an "A-Lister"?

You hear it everywhere on tabloid shows - a new rating system they've invented to tell us who THEY think are the most valuable Hollywood so-called "stars" - to influence us to believe these people are greater than they are. When I hear them refer to certain people as "A-listers," sometimes I'm taken aback. What IS this new rating system tabloidists have invented? Is it something political? Are public relations firms PAYING for these so-called ratings? I'd like to know! Because the "A-listers" they chase after and talk about non-stop appear to be average, mediocre people with very little talent, with one differentiating factor: something they've done has caused them to be chased by paparazzi; they're famous for being famous. Do these so-called "A-listers" have any proven track record or any great accomplishments? For the most part, I've observed that those who garner all this attention haven't the accomplishments commensurate with the attention they're getting. So what is this media-driven ratings system? I wonder.