Bee swarms come in all shapes and sizes. They are mostly a product of the spring, when it is finally warm enough for honey bees to leave the mother hive. For the most part, they are not dangerous and should never be sprayed with pesticides!
Swarming is a natural behavior, and is the colony’s method of propagation. Bee colonies — which neither perish annually with the onset of cold weather, nor hibernate — are alive and well (hopefully) all winter long. The queen bee does slow or even stop laying eggs until after the winter solstice, but as soon as the days start to get longer, she starts laying again. By the time the warm weather hits, a colony’s home can be very crowded!