Hornets are wasps, closely related to and resembling yellowjackets. These social insects construct hives by chewing wood into a papery construction pulp. Hornets are often considered pests, particularly when they nest near humans, because they will defend a nest aggressively if they feel it is threatened.
As noted, Hornets are extremely aggressive and will sting any human or animal that gets too close to the nest. For this reason, a pest control professional should always be consulted if a hornet’s nest is spotted near your home.