There are three main types of antennas:

<ul>
<li><b><a href="http://en.wikipedia.org/wiki/Omnidirectional_antenna">omnidirectional antennas</a></b> (AKA an "omni") &mdash; 360&deg; coverage [in the horizontal plane]
<li><b><a href="http://en.wikipedia.org/wiki/Directional_antenna">directional antenna</a></b> &mdash; an antenna designed to radiate mainly in one direction.  They're especially good for fixed point-to-point communication.
<li><b><a href="http://en.wikipedia.org/wiki/Sector_antenna">sector antennas</a></b> &mdash; something in between directional and omnis; designed to be somewhat directional, but not point-to-point