The basic difference between AM and FM radio is contained in their names; AM stands for amplitude modulation while FM stands for frequency modulation. The way in which radio waves are manipulated to produce sounds in a radio creates differences which affect the quality of the signal.
Both types of reception are subject to slight variations in radio wave amplitude. In AM radio, this affects the quality of sound and produces static. FM radio is found from 88 Mhz to 108 Mhz while the AM band is found between 535 to 1605 Khz.
FM waves are much shorter than AM waves. This means that FM waves and reception are much less affected by the presence of metal structures such as bridges and elevators. AM waves typically carry much farther in the absence of such structures however.