Drone bombs became one of the favourite attacks by IS militants during the Battle for Mosul. They are a remarkably effective terror weapon, as unlike car bombs, they aren't stopped by walls or barriers. By all accounts they weren't actually particularly effective at actually causing damage.

They were basically dropping 40mm grenades (the kind used in under-barrel launchers like an M203) with fins attached.

Here's a video released by IS showing them purportedly blowing up a humvee with a grenade dropped from a drone: https://www.bbc.com/news/av/world-middle-east-39092883/so-ca...

Here's an article from the BBC with pictures of captured drones: https://www.bbc.com/news/technology-38663394