Now radio waves don't normally travel in just one direction, they ravel outward like ripples in a puddle. But in terms of wave propagation "outward" is one direction. Radiowaves are electromagnetic waves, both AM and FM waves are polarized. This starts at the antenna. The antenna is basically a transducer it converts electrical currents into electromagnetic waves. So here at the physical antenna, polarization refers to the orientation of the electric field or E-Plane. (E-Plane just refers to the wave in reference to the surface of the Earth. ) The term H-Plane will also come up. That's the direction of the maximum magnetic field. More here.
For example in a linearly polarized antenna the "H" plane lies at a right angle to the "E" plane. For a vertically polarized antenna, the H-plane usually coincides with the horizontal/azimuth plane. For a horizontally-polarized antenna, the H-plane usually coincides with the vertical/elevation plane. OK More diagrams here.
Linear Polarization: The polarization of the magnetic field vector to a given plane along the direction of propagation.
Vertical Polarization: This is used for AM & FM. Vertical polarization is most often used when it is desired to radiate a radio signal in all directions.
Horizontal Polarization: This is used for Television. The polarization of the magnetic field vector to a given plane primarily in the vertical plane sending much of it's energy into the ground and outer space, but it is more noise-resistant.
Circular Polarization: (There is both Left and Right circular polarization) The electric field vector of a constant magnitude rotates in the x-y plane.
Elliptical Polarization: The polarization of magnetic field vector such that the "tip" of the variable magnitude of the electric field vector describes an ellipse in a fixed plane intersecting, the direction of propagation. Linear propagation is a subset of this, and true elliptical polarization does not occur in radioland.