A swarm is the way bees multiply. (think of a bee colony as an individual entity with many components)

Once a swarm queen has hatched, generally the old queen will leave with a large proportion of the hive workers that have gorged themselves on honey to start a new colony.

When the hive is very strong, or in an undesirable place, they can swarm out. Several queens are raised at once and the original queen does not kill the many cells produced.

Each queen will leave the hive with ever decreasing numbers until there are virtually no bees left.

Swarms usually occur a day or 2 before bad weather in order they can build up new comb while the weather is unsuitable for flying, that way when the weather clears there is comb ready to put stores in.

Swarms normally occur September through November but depending on the season August to January. Swarms may occur after January but they are often too small and its too late in the season to for them to survive as a colony. In this case the small swarm will be put on top of another swarm caught earlier to boost the bee numbers.