Definitions
Effective Radiated Power (ERP)
ERP in watts is measure of the apparent output of a transmitter and it's antenna system. A high ERP does not guarantee a large range as the height of the transmitter above the local terrain is at least as important as transmitter power. High power transmitters on prominent hills or mountains will often serve most customers in an area with weaker transmitters used to fill in specific problem areas.
See: http://en.wikipedia.org/wiki/Effective_radiated_power
Omnidirectional Antennas
An omnidirectional antenna emits power equally in every direction in the horizontal plane (they do not emit much power directly up or down). Given good line of sight a transmitter with an omnidirectional antenna will serve areas in a circle around it.
See: http://en.wikipedia.org/wiki/Omnidirectional_antenna
Directional Antennas
A directional antenna emits more power in one or more directions than in others. Broadcast transmitters will use a directional antenna to aim the signal at their intended area or areas of service. Directional antennas increase the Effective Radiated Power for the areas that they target without requiring a more powerful transmitter. On the other hand if you are not in the intended area of service the signal is likely to be weaker than if the antenna was omnidirectional.