Polarization

Another important quality of electromagnetic waves is polarization. Polarization describes the direction of the electrical field vector.

If you imagine a vertically aligned dipole antenna (the straight piece of wire), electrons only move up and down, not sideways (because there is no room to move) and thus electrical fields only ever point up or down, vertically. The field leaving the wire and traveling as a wave has a strict linear (and in this case, vertical) polarization. If we put the antenna flat on the ground (horizontal, we would find horizontal linear polarization.

Figure : Electric field and complementary magnetic field components of an electromagnetic wave. Polarization describes the orientation of the electric field.

Linear polarization is just one special case, and is never quite so perfect: in general, we will always have some component of the field pointing other directions too. The most general case is elliptic polarization, with the extremes of linear (only one direction) and circular polarizations (both directions at equal strength).
As one can imagine, polarization becomes important when aligning antennas. If you ignore polarization, you might have very little signal even though you have the strongest antennas. We call this polarization mismatch.