Cinematic Expression - Focal Length
Focal Length of the Lens can be expressed as:

Distance between the centre of the lens (nodal point within a lens) and the Film Plane, when Lens is focused at infinity.

Focal Length is related to by numbers (millimeters and/or inches). When we speak about 80mm Lens, it means that when this lens is focused at the point at
infinity then the distance between the centre of the lens (nodal point) and the image of the focused point on the film plane will be 80mm.
Primarily, there are three elements of Focal Length that are of cinematographer's concern:
1. Field of View
2. Amount and Quality of Perspective Distortion
3. Depth of Field  
In this respect to the above properties of  Focal Length all lenses are grouped in three categories: Wide (Short). Normal and Long (Narrow).

The so called "Normal" (50mm Lens in 35mm film format) is called so not because - as popular opinion dictates - it resembles field of view of human vision
(it does not), but rather because it introduces the least amount of perspective distortion.
Lenses of shorter than Normal focal length are called Wide (Short) and lenses of longer than Normal Focal Length are referred to as Narrow (Long).
It is a size of Film Format (size of the Negative and / or Size of  the Digital Sensor) that determines the "Normal" Focal Length of the Lens...  And so, in 35mm Film
Format (Full Frame Digital Cam Sensor)  Normal Lens is 50mm Lens, but in 16mm Film (equivalent of 1/2" Digital Sensor)  "Normal" is 25mm Lens.
Further implications of the Focal Length are:
Depth of Field and Perspective Distortion.
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         Long (Narrow) Angle  Lens                                                   Short (Wide Angle) Lens