Bees use swarming to reproduce the species. As the colony grows and becomes too big for the current space, they will grow a new Queen and just before it is born, the existing Queen and a good proportion of the adult bees will fly in what can at first be an alarming, buzzing cloud. They are really not too dangerous in this state as they have nothing to defend and are well fed for the coming journey to find a new home.
They like this car 
Swarm on a branch 
Inside a compost bin 
Running a swarm into a hive