Wasps are any number of insects that are mainly part of the superfamilies Vespoidia or Sphecoidea that may live alone or, more commonly, as part of a communal and social group. The term wasp is a general term that both hornets and yellowjackets are part of. In other words all hornets and yellowjackets are wasps, but not all wasps are hornets or yellowjackets.
Both yellowjackets and hornets can deliver a very painful sting and come in different variations of black and yellow. Yellowjackets tend to have more pronounced black and white stripes running across their bodies than do hornets. These insects share many common traits but are also very different. These differences, over time, have led to two distinct species. Hornets grow to a size of .75 inches (1.9 cm). Yellowjackets are slightly smaller.
One of the biggest differences between yellowjackets and hornets is their choices of nesting locations. Yellowjackets are often found nesting underground in areas that have been hollowed out by rodents or other animals. Hornets, on the other hand, usually stay above ground, nesting in trees, shrubs and perhaps under eaves of homes. They both build nests out of wood materials ground up into paper.
The aggressiveness of the two species accounts for another major difference between yellowjackets and hornets. Yellowjackets are usually much more aggressive, especially around their nests. However, yellowjackets and hornets both can defend their nests and as their numbers grow, do so with increasing ferocity as their numbers increase. Therefore, the late summer and fall, when their numbers are at their highest, are the times they cause the most problems for humans.
Another thing that attracts yellowjackets is sweet-smelling aromas. This is often why they are a problem in outdoor areas where people are eating. They are also attracted to the smell of anti-freeze/radiator fluid and may often be found searching near the front of parked vehicles. Again, because they can be quite aggressive, encountering yellowjackets in any situation offers a chance to be stung, but generally they are not interested in people unless they perceive a direct threat.
However, despite the problems they can cause, both yellowjackets and hornets may have a net benefit to humans. They are predatory and often hunt insects deemed to be greater pests than they are. Unless there is a nest in close proximity to a home, or other type of heavily trafficked building, the best thing to do is leave them alone.