The Answer:
Essentially, the term "hornet" (like "yellow jacket") refers to
a kind of wasp that
builds a large papery nest.
One type of hornet, the baldfaced hornet, is black and white and
about ¾" long. They tend to build mottled grey nests in trees or
shrubs. Occasionally, they will build nests under roof overhangs, in
attics, crawlspaces, and walls, or under decks or porches. The nests,
which are generally pear-shaped, are constructed of a paper-like
material formed from chewed wood. These hornets like to feed on flies
and other insects." long. They tend to build mottled grey nests in
trees or shrubs. Occasionally, they will build nests under roof
overhangs, in attics, crawlspaces, and walls, or under decks or
porches. The nests, which are generally pear-shaped, are constructed
of a paper-like material formed from chewed wood. These hornets like
to feed on flies and other insects.
Yellow jackets, by contrast, are only the size of house flies,
with distinct yellow and black markings. They build a very similar
nest, but it is tan in color, smaller in size than a hornets' nest,
and is usually found in an underground cavity, such as an abandoned
rodent burrow. Occasionally, yellow jackets will nest in attics or
walls. Yellow jackets are scavengers, and are usually encountered at
cookouts and around dumpsters and trash barrels.
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—The Editors