An antenna is an electrical device designed to transmit or receive radio waves or, more generally, any electromagnetic waves. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, radar, and space exploration. Antennas usually work in air or outer space, but can also be operated under water or even through soil and rock at certain frequencies.
Physically, an antenna is an arrangement of conductors that generate a radiating electromagnetic field in response to an applied alternating voltage and the associated alternating electric current, or can be placed in an electromagnetic field so that the field will induce an alternating current in the antenna and a voltage between its terminals.
An electromagnetic field is a physical influence (a field) that permeates through all of space, and which arises from electrically charged objects and describes one of the four fundamental forces of nature - electromagnetism. It can be viewed as the combination of an electric field and a magnetic field. Charges that are not moving produce only an electric field, while moving charges produce both an electric and a magnetic field.