The BBC has the story. Female suicide bombers from Dagestan have become increasingly common. “Black Widows”, as the article alludes. Volgograd was once called Tsarytsin, and beginning in 1925, Stalingrad.
There was speculation elsewhere that the woman had carried a hand grenade aboard. The above photo is reportedly the detonation, and if so, certainly appears to show a device much larger than a hand grenade being detonated inside the bus.
Russia has its hands positively full of Islamist extremists. At least they have the good sense to call them what they are.
Update-XBradTC: Here’s video of the attack. Concur that it’s a good deal larger than a hand grenade.