Yellow jackets are a small, nasty wasp that live in the ground. Most people that see them assume they are bees, because they are yellow and black striped and about the same size as bees. If you looked closely, though, you would notice that they have no hairs on their body, which makes them wasps.
The name also confuses people because we usually grow up calling the yellow and black/brown paper wasps yellow jackets. Those are, in fact, paper wasps. TRUE yellow jackets are ground-living wasps that make their nest out of cellulose material and make it look like a paper mache. If its a stinging insect coming out of the ground, living in a colony, its probably a yellow jacket. Bees do not live this way.
This time of year, they seem to be thriving. Actually, summer through the warmer parts of fall, they thrive! They are very common nesting in Asiatic Jasmine - probably because it provides a bit of shelter for them.
These are nasty wasps, so be careful around them! Most people don't know they have them until they are mowing or weed whacking and they get attacked - and it can be bad! They get that way when they feel the vibrations of the mower and come out to defend their colony. It really makes maintaining your yard difficult.
If you've got them, you can control them using a pesticide in a dust formulation. Find that hole they are coming in and out of, and sprinkle some dust in and just around the hole. When they enter they will grab some on their body and spread it around inside. Depending on how big that nest is, you may have to do this a few times. Be VERY careful when you do, because they can be pretty cranky. Be ready to run and try to do it around dusk when they are less likely to attack.