It mostly refers to early and mid nineties girl bands with political and prodominantly feminist music. It can encompass many bands as i've seen people go as far to say the above mentioned would fit. I really don't think they do though. I would keep saying what it is but I like audiogalaxy's definition so I will just put that right here:

Riot Grrrl
First arising in the late ‘80s through a network of new bands, labels, and fan-published ‘zines, Riot Grrrl is typified by a loud, abrasive Punk Rock sound and attitude, and aggressive feminist politics. Drawing inspiration from early girl-punks like the Raincoats and Yoko Ono (believe it!), Riot constructed a new ethos of empowerment and creation, making it OK for girls to scream, rock, and do it themselves. Hole is the best-known Riot Grrrl band, but Bikini Kill and Babes in Toyland are far more legitimate.