In a short mobile video clip which appears to have been taken by locals at seen of the murder, the girl is seen being ambushed on her way home by a group of up to 1000 men who were waiting for her to return; the men killed her in the most brutal way possible, by throwing large stones on her head. The following clips show that while she is alive and crying for help she is taunted and kicked in her stomach until someone finishes her off by throwing a large stone on her face..3gp video files (warning: contains extremely graphic violence) and account at kurdishaspect.com. I see related reports at Daily Mail and Foxnews.com, but there's nothing online at sources I'd consider more reputable -- so I don't know how reliable the reported details of this incident are.
From the clips it appears that the girl was first stripped naked to symbolize that she had dishonored her family and her Yezidi religion. She is lying on the road naked while her smashed face is covered with blood and still breathing.
UPDATE: BoingBoing reader Derek says,
AP mentions a similar story of the incident, so what happened seems to be established. As a bit of an update, the article is mainly about how a gunman executed a bus full of Yazidis in response to the stoning of the girl. Vicious circle.At least 23 have been killed in retaliatory violence: Link to Amnesty International statement. Peaceful protest rallies against the violence took place in Erbil: Link to blog account.