Swarms are the method by which honey bee colonies reproduce. 20-50% of the colony leaves the hive with the old queen, landing usually within a couple hundred feet of the hive they came out of. Most of the time they end up on tree branches, but sometimes they'll decide to land elsewhere, such as on mailboxes, cars, fences, door handles or anything else you can think of. This is only temporary, though, as the colony sends out scouts to find a new, permenant home. This is often a tree cavity, but anywhere the cavity is large enough and dry is suitable. The walls of houses are many times chosen by honey bees as safe, dry places in which they can thrive.
They are usually completely docile and uninterested in stinging you or your family when they swarm, so do not be too alarmed. The bees just want to find a new home. When we arrive we'll survey the situation, and if the bees can be accessed safely we'll drop them into a bee hive, wait a few minutes to confirm we got the queen, and then we'll seal them up and take them to one of our apiary sites!