OmnidirectionalOmnidirectional antennas are design to transmit and receive signals in all directions equally (although less so along the axis of the antenna). The rubber-ducky antenna is a common example of an omnidirectional antenna.
Antennas are passive gain devices, in other words they concentrate a signal in certain directions but the overall power output is never more than the input power. Antenna gain is compare to a theoretical isotropic antenna which radiate signal in a perfect sphere. A dipole omnidirectional antenna will generally have a gain of about 2.15dBi.Its radiation pattern will be squashed in the vertical plane (usually desibed as doughnut shaped).
Directional AntennasDirectional antennas focus the energy and therefore have a higher gain than omnidirectional antennas. They are used for indoor areas such as hallways and in warehouses to direct the signal between racking.
Antennas are designated by their beamwidth. This describes how focused the energy is in the vertical plane and is calculated as the range between the half power points i.e. the points where the power output is half that of the maximum power.