If all the displacement vectors of the oscillating particles in a transverse wave are in one fixed plane, then the wave is plane-polarized (linearly polarized).
Wave is circularly polarized when the displacment vector rotates as seen by an observer toward whom the wave is approaching/leaving.
Wave is unpolarized (e.g. natural light) if the polariztion at one point changes rapidly (for every 1 ns in natural light) and in a completely unpredictable fashion.
Polarizer is a device which transmits only the component of polariztion parallel to its transmission axis. By using a polarizer, the plane of a plane-polarized wave can be changed; a circularly polarized or unpolarized light can be changed to plane-polarized.
No wave can pass through two polarizers whose transmission axes are crossed perpendicularly. However, when a third polarizer is inserted between them with an angle not parallel to either one, some wave can finally pass through.
Malus' law: The intensity (proportional to the square of amplitude) of the wave passing through a polarizer (I) is the intensity of the incident plane-polarized wave (Io) times the square of the cosine of the angle between the transmission axis of the polarizer and the incident polariztion (q), i.e. .
When a circularly or unpolarized wave is incident on a polarizer of whatever the orientation of transmission axis, the wave intensity passing through is always one-half of the incident one (after averaging over many random changes in the case of unpolarized wave).
You may drag the whole set-up to change the angle of viewing.