There's quite a lengthy and interesting article on Baba Yaga in Wikipedia. The first paragraph:

In Slavic folklore, Baba Yaga is a supernatural being (or one of a trio of sisters of the same name) who appears as a deformed and/or ferocious-looking witch. Baba Yaga flies around in a mortar, wields a pestle, and dwells deep in the forest in a hut. In Russian folkloric myths her house is described as standing on chicken legs (or sometimes a single chicken leg). Baba Yaga may help or hinder those that encounter or seek her out. She sometimes plays a maternal role, and also has associations with forest wildlife. According to Vladimir Propp's folktale morphology, Baba Yaga commonly appears as either a donor or villain, or may be altogether ambiguous.

Mussorgsky gave us some great music about Baba Yaga in Pictures at an Exhibition. Here's a sample, courtesy of Emerson, Lake, and Palmer:

https://www.youtube.com/watch?v=xMdwIP9vRWs

I think I prefer the Mussorgsky-Ravel orchestration:

https://www.youtube.com/watch?v=sxTLk8gWJHg

-Joe-