Yellowjackets and Hornets are closely related to the familiar honey bee, but have decidedly different lifestyles. Yellowjackets are more likely than honey bees to sting without provocation, perhaps because yellowjackets don't leave their stinger in your skin, and can thus inflict several painful venomous stings. Yellowjackets live in gray papery nests found below ground, suspended in trees, or within walls. Like honey bees, each nest is controlled by a single queen who lays eggs that hatch into hundreds, or even thousands, of workers by the end of summer. Thus, an extended warm fall, like we have had this year, results in a large buildup of yellowjackets.
Populations rapidly decrease with the first cold weather.
Yellowjackets aren't all bad. In fact Rodale, in the Encyclopedia of Organic Gardening, asks us not to automatically destroy yellowjackets, or their gray paper nests, because these insects are excellent pest predators. I suspect they must also foray in lawns for various pests. However, if the nests are in a place frequented by people, or if a family member is highly allergic to stings, control is necessary. Avoid attracting yellowjackets to your yard in the first place by controlling their access to garbage.