The document discusses microstrip lines. Microstrip lines consist of a conducting ground plane, dielectric substrate, and a microstrip conductor with a thickness measured in micrometers and a height in millimeters. Microstrip lines have a width-to-height ratio between 0.1 and 10 and propagate electromagnetic waves in the z-direction. While the waves are quasi-TEM, fringing effects cannot be ignored due to the waves not being purely transverse electromagnetic. The document outlines characteristics of microstrip lines such as relative permittivity, effective dielectric constant, wavelength, velocity factor, and characteristic impedance.