Yellow jacket or yellow-jacket is the common name in North America for predatory wasps of the genera Vespula and Dolichovespula. Members of these genera are known simply as "wasps" in other English-speaking countries. Most of these are black-and-yellow; some are black-and-white (such as the bald-faced hornet, Dolichovespula maculata), while others may have the abdomen background color red instead of black. They can be identified by their distinctive markings, small size (similar to or slightly smaller or larger than a honey bee), their occurrence only in colonies, and a characteristic, rapid, side to side flight pattern prior to landing. All females are capable of stinging. Yellowjackets are important predators of pest insects.

A typical yellowjacket worker is about 12 mm (0.5 inches) long, with alternating bands on the abdomen while the queen is larger, about 19 mm (0.75 inches) long (the different patterns on the abdomen help separate various species). Workers are sometimes confused with honey bees, especially when flying in and out of their nests. Yellowjackets, in contrast to honey bees, are not covered with tan-brown dense hair on their bodies and lack the flattened hairy hind legs used to carry pollen. Yellowjackets have a lance-like stinger with small barbs and typically sting repeatedly, though occasionally the sting becomes lodged and pulls free of the wasp's body; the venom, like most bee/wasp venoms, is primarily only dangerous to those who are allergic, unless a victim receives a large number of stings (main article: Bee sting). All species have yellow or white on the face. Mouthparts are well-developed for capturing and chewing insects, with a proboscis for sucking nectar, fruit and other juices. Nests are built in trees, shrubs or in protected places such as inside human-made structures (attics, hollow walls or flooring, in sheds, under porches and eaves of houses), or in soil cavities, mouse burrows, etc. Nests are made from wood fiber chewed into a paper-like pulp. Yellowjackets have two antennae and two wings. These two wings are distinctive because they fold in half length-wise.

Due to their aggressive behavior, including stinging, many other insects exhibit mimicry of yellowjackets; in addition to numerous bees and wasps (Müllerian mimicry), the list includes some flies, moths, and beetles (Batesian mimicry).

Yellowjackets' closest relatives, the hornets, closely resemble them but have a much bigger head, seen especially in the large distance from the eyes to the back of the head.

References:
[1] Text from Wikipedia, the free encyclopedia: Yellow Jacket (Wasp)
[2] Photo from Wikipedia, the free encyclopedia: Yellow Jacket (European Wasp)