a swarm is the colony halved. the old queen leaves with perhaps half the workers and a quantity of honey she has had them carry in their stomachs as portable savings. they cluster on a branch — sometimes a fence, sometimes the lintel of a strange door — and wait while scouts investigate possible homes.
the cluster is calmer than it looks. without comb to defend, the swarm is famously unwilling to sting. an old country trick is to slide a sheet beneath it and lower the branch into a waiting box. the bees, finding themselves indoors, agree to live there.