Polarization

The direction of the electric field component of an E-M wave defines its plane of polarization
Most sources of light produce waves with electric fields at all orientations perpendicular to the direction of travel: this mix of waves is unpolarized
A few sources of light produce waves with their electric fields preferentially oriented in one particular direction: we call this polarized light
Unpolarized light can be polarized by passing it through a special filter which absorbs waves which have their electric fields perpendicular to its axis of transmission
Two polarization filters can block all light from passing through them if they are oriented with axes perpendicular to each other
Light can also be polarized if it scatters or reflects off molecules or a surface
Malus' Law describes the intensity of polarized light which passes through a second polarizing filter, oriented at some angle to the light's plane of polarization
Liquid crystal displays (LCDs) are one very common and important application of polarization.

Here's one example of the difference a polarizing filter can make:

Image courtesy of blueshark

Take a look at a pair of images, one taken through a polarizing filter, the other without any filter.