Antennas can be horizontal, vertical or circular, which is a combination of horizontal and vertical. Horizontal polarization is effective when transmitting to table top radios or stereos with roof mounted TV type antennas.
Vertical polarization works best with boom boxes and autos.
Circular polarization is the most commonly used FM antenna as it is effective in reaching all types of receivers. In the US the Vertical component of a signal is not allowed to exceed the horizontal. Antennas can have from 1 to about 8 bays. The number of bays increases the gain of the antenna, requiring less transmitter power to achieve a particular transmitted power. However, each bay added takes up about 10 feet of tower space, which is often paid for by the foot, so common setups today use 2 to 4 bays.