This is what a swarm of honeybees looks like - not to be confused with bumblebees or wasps, which don't swarm in this manner. Swarming is the way honeybees multiply to create new nests - during spring and summer, new queen bees hatch and leave the old nest along with a number of worker bees and drones, in search of a new home. On the way, they usually rest up wherever they can for anything up to a few days, while the "scout bees" go off in search of the perfect permanent home (a hollow tree or suchlike is their preferred natural habitat).
Swarming bees are interested in only one thing - finding a new home - and are extremely unlikely to be aggressive as long as you leave them in peace. They will have filled up their stomachs with honey before leaving the parent nest, and consequently are usually in a fairly mellow, drowsy mood (just like us humans after a hearty Sunday lunch!)