Yagi Antenna: A directional antenna, meant to propagate signal in a specific direction rather than to spread it all around.  These are commonly used in bridging applications, where two wireless bridges are separated by a great distance and direct their signals at each other to establish a point-to-point link.  Yagi antennas are generally stronger than patch antennas, which are a different kind of directional antenna.

http://en.wikipedia.org/wiki/Yagi_antenna

Omni Antenna: An antenna that covers 360° around itself, providing service to all clients in an area.  These are almost always used when supplying coverage to clients, but they can also be used in point-to-multipoint bridging applications.  The coverage is not a sphere, but looks more like a donut, with coverage 'gaps' directly above and below the antenna (assuming it's oriented vertically).

http://en.wikipedia.org/wiki/Omnidirectional_antenna#Short_dipole

Dipole Antenna:  This is essentially an omnidirectional antenna, but it's commonly referred to in wireless as a refrence point to relate other antennas.  The reference point is supposedly a theoretically perfect antenna, with signal being generated from a single point in space with a perfectly spherical coverage.  Since it's theoretical, this kind of antenna doesn't exist, but all real antennas can be compared against this and given a value.

http://en.wikipedia.org/wiki/Dipole_antenna#Dipole_as_a_reference_standard

Also, see this document:  http://www.cisco.com/en/US/prod/collateral/wireless/ps7183/ps469/product_data_sheet09186a008008883b.html

Look about 2-3 pages down until you see the section called "Type of Antennas".