If that's a swarm of honeybees, then they're actually all engorged with honey and won't likely sting you. If you call your local beekeepers society, they'll gladly take them away.

What happens is when a hive gets too large, the queen will lay a special queen egg and then take off. Over half of the bees in the hive suck up as much honey as they can and take off with her (imagine thousands of bees flying around in the air like mad). This is called swarming. They rendezvous nearby, usually on a tree branch, and look exactly like the pic you've taken. While they're in this huddle, scout bees are sent out to check out potential new hive spots. It can take days for a good spot to be found and the bees leave to go there (which can be a good distance away).

When the huddled bees are on a branch, beekeepers will come with a cardboard box, set it up beneath the group, and smack the limb. The bees fall into the box, they close it up, and take it off to a prepared hive. The bees are damn near comatose on honey reserves and don't put up a fight. They open the box near a prepared hive, the scouts will find it easily, and they'll likely move in.

It's a really cool phenomenon (swarming), and one that beekeepers work hard to prevent in their own hives since the colony left behind may die off (especially if the new virgin queen decides she needs to swarm, further thinning the hive's numbers). If the keepers feel the colony is getting too large, they'll add more supers so they don't get overcrowded. In winter, the numbers will thin out and the supers can be removed.

It's a fascinating hobby. My wife was going to start a hive prior to us getting pregnant. Now we'll probably wait until our daughter is a few years old and can understand to not bother the hive.