Think Progress has a decent piece on barrel bombs, which it also calls "flying IEDs," among other things.
That second phrase is apt. Basically, it's a barrel made out to be a large-scale version of the Tsarnaev brothers' backpack bombs at the Boston Marathon. They're cheap and easy to make. But, as the piece notes, they're indiscriminate in their killing, too.
However, it's all in a word. The Luftwaffe's WWII butterfly bomb was the forerunner of a host of US cluster bombs. And cluster bombs of various sorts have been known to kill civilians decades after having been dropped.
You know, just like mines are evil but the US has yet to sign the Ottawa Treaty to ban antipersonnel land mines.