Well we don't know exactly who they are (because of their disguises), but we do know that they are all women artists who, since the mid-1980s, have been protesting against racism and sexism in the art world.
Guerrillas – NO not GORILLAS! This isn't a wildlife website! – are rebels who do disruptive, subversive things to make people aware of their beliefs. This is why the group decided to call themselves the Guerrilla Girls.
The Guerrilla Girls hide their real identities by calling themselves names of great women artists from the past like Frida Kahlo and Käthe Kollwitz, and by wearing masks when protesting or fly-posting their posters. (They were originally going to wear ski masks to hide their faces but when one of their gang accidentally spelt guerrilla wrong and wrote gorilla, they thought it would be funny to wear monkey masks instead. We think they look pretty cool!)
They are best known for the posters they create and put up in places where they shouldn't, like on walls next to art galleries.
The Guerrilla Girls use real facts and figures in their posters to make their point, they say they are 'fighting discrimination with facts'. But as well as facts they use humour to change people's minds. One journalist described them as 'quippy as well as lippy'.
Look at this poster:
It's a report card (like you get in school) where they have marked and commented on the performance of art galleries. One gallery is told that it is 'boy crazy' another that it is 'not paying attention'.
(Hahaha! You rock Guerrilla Girls!).