Light as waves
Electromagnetic energy, of which visible light is a small component, can be described as having a wave form with two vector components, E: electric and H: magnetic, vibrating in planes orthogonal to the direction of propagation (Z), and 90° apart. The amplitude (A) of each vector oscillates in a sinusoidal pattern. The distance between peaks is the wavelength.

The electric (E) vector is the component of light that interacts with matter in a way that is important in microscopy. If, for example, the E vector encounters certain crystalline atomic structures, the amplitude may completely diminish (at 90°) or pass unhindered (0°).

Polarized light
For any given ray, the vibration of the light vector (E) can assume one of three orientations: 1) Planar, vibrating along a geometric plane; 2) Circular: amplitude precessing equally around the axis of propagation; 3) Elliptical: amplitude precessing unequally about the axis of propagation. Here is a Wikipedia page explaining the difference. Here is another page explaining in great detail the concept of polarization.

While any single ray exhibits a Plane of Polarization, light from most luminous objects (e.g., a lamp) arise from a very large number of points. Thus a beam of light consists of rays vibrating in every possible orientation. Light composed of these rays is said to be non-polarized. Light vibrating in a single plane is said to be Plane Polarized. Surface reflections (at certain angles: Brewster angle), light through polarized sunglasses, and laser light is polarized.

Graphical convention:

Arrows indicate the plane of polarization:

A: End view, unpolarized
C: Side view, unpolarized

B: End view: Plane polarized
D1: Side view: Plane polarized vibrating in the plane of the screen
D2: Side view: Plane polarized vibrating orthogonal to the plane of the screen.