Learn the facts about swarming 
There are about 250 species of Solitary bees and Bumblebees and one specie of Honeybee in the UK. Solitary bees and Bumblebees live in small colonies. Solitary bees may only have a dozen members of a colony and Bumblebees as many as 300 which is a really big colony. Contrast those numbers with honey bees. A small honey bee colony might be 20,000 strong whereas a big colony might be closer to 80,000 individuals. Given the difference in numbers of bees in a colony the number of bees in the air will give an indication of the type of bee that is swarming. 
A honey bee swarm cannot be mistaken. A swarm will be seen as a black cloud of bees. There will be anywhere between 15,000 and 40,000 in the air at once. On the other hand a bumblebee swarm is not in fact a swarm at all. Bumblebees do not swarm. One might see many of them, 20 odd bees buzzing round the entrance to a hive. Depending on the species these will be new workers locating the hive before going off to forage or Drones simply flying to show off their strength. 
A new comer to the UK is the Tree Bumblebee. It is a European invader and is the bee I as a beekeeper get calls about. They have a liking for bird boxes and eaves of houses. It is quite a big bumble and a noisy flyer. Bumbles are completely harmless unless seriously provoked when they will sting or bite. A Tree Bumblebees nest might contain 150 individuals. The colony will naturally die in late summer when the Queens leave the hive to find a place to over winter. The drones and workers simply die. It is the bumblebee way. Find out more from the Bumblebee Consevation Trust 
Whilst the new Queen honeybee develops some of the foraging worker bees look for a new home. These are the bees you will see floating round your house. They are looking for a dark, dry, well ventilated cavity that is not to too big or too small. The foragers will return to the colony to communicate the potential new home, then different bees will check out the potential new home to confirm it meets the colony’s needs. 
There is a point in the new Queens development where the queen cell is capped to allow the larvae to pupate which is the final stage of any insects development cycle. When this cell is capped this is the trigger for the bees to depart the old hive. The incumbent Queen will have had her food reduced by the worker in the days ahead of the swarm event to slim her down for flight. On a sunny and warm day, usually 24/48 hours after the queen cell has been capped the incumbent Queen, some are the drones and all the forager bees leave the hive en masse. This is what we call a swarm. 
The parent colony from which the swarm has emerged contains eggs, larvae, and sealed brood together with a Queen cell and all the nurse and house bees. It is roughly a 50/50 spilt of bee numbers. Remember the foragers have been collect nectar and pollen so the hive is well stocks to provide for the colony until the nurse bees grow into foragers. In due course the eggs become larvae, the larvae sealed brood and the sealed brood emerges as bee to take up nursing duties. The Queen in the cell will also emerge and after a few days then goes on mating flights. When she decides she is fully mated she stops the mating flights and starts to lay eggs thereby continuing the colony. 
The first swarm of the year is usually the largest and is called the prime swarm. If the colony creates further swarms, which is a possibility, these are called casts. Primes and casts act in the same way. They fly up in to the air en masse, surrounding the Queen. Once the swarm is assembled in the air they move off usually to an adjacent tree or bush. In the cities the tree might be replaced with the car, bike, porch or pretty much anything they fancy alighting upon. This is a staging post or a bivouac. The bees might stay there overnight. In the meantime the foragers check out the new potential homes. Once a new homes is selected. The bees then fly off again en masse and lead the Queen to the new home. Once inside the new home the workers start to produce wax to make honey comb into which the Queen will start laying eggs to establish a new colony. 
Depending on the strain of Bee, it is during swarming that bees are at their most benign. They have gorged on honey prior to leaving the parent hive and are intent on moving into the new home. A prime swarm might contain 20,000/30,000 bees. This can be very intimidating when they are all flying. Should a swarm alight in an inconvenient place i.e. near people, the best advice is to get out of the area. Try not to wave your arms about or swat the bees as you leave. They react to the movements in the only way they can and that is to sting. Remember they have a Queen in an exposed condition to watch out for. On a hot day they might stay in the bivouac for a few hours before flying off again or they might stay overnight. As the old saying goes: 
A swarm in May is worth a load of hay, 
a swarm in June is worth a silver spoon, 
a swarm in July is not worth a fly.