Some very useful information on swarms from the NSW Department of Primary Industries.
Here is an excerpt.
Swarming is part of the natural reproductive life cycle of honey bees.
The swarming season is usually between September and December but on rare occasions colonies may swarm at other times.
The old queen, together with about half of the bees from the colony, leave the hive and cluster on a nearby object such as a fence or a small shrub. When scout bees (worker bees) have found a permanent nesting site the swarm will move to this site and establish a new colony.
Swarming bees are generally not inclined to sting provided they are left alone, but there are precautions that should be taken.