Electronic currents create magnetic fields that radiate away from the currents. At low frequencies, this energy is mainly re-absorbed by the current, as the direction of current changes. As the frequency is increased, however, proportionately more of the energy is lost to this magnetic field, which in turn creates an electric field. At sufficiently high frequencies, most of the energy at any given moment is stored in this field. At this point, it makes more sence to transmit this electro-magnetic field, than the current itself.
Waveguides can have a circular or a rectangular cross-section.
They can have other shapes as well, if motivated for some reason or other. In the beamcar-context, we will only concern ourselves with rectangular waveguides. Depending on their shapes and dimensions, they can transmit certain frequencies in what is termed modes. These modes are also determined by the fact that waveguides are made from electrically conducting material, often aluminum. Thus, the electric field at the walls of guides are always zero.