For safety reasons (human and pets), electrical systems in the USA have a wire connected to earth ground at the service entrance. This “ground” wire is run along with the two current carrying wires. Most electrical equipment is electrically connected to this wire when a three wire plug is used to connect the equipment to the electrical power receptacle. This third, non current carrying conductor which is connected to earth ground at the service entrance of a building is often referred to as “ground”. Receptacles were not required to carry a ground wire until the 1960’s. Therefore you may live in an older home that does not have a ground wire attached to the receptacles.