A pixel is an image point in a digital image and defines a single color value. Pixels are used both in the display (screen, beamer) and when capturing (camera, scanner) an image.

Pixels are basically pure digital color values which can be put together in a grid to represent an image. The pixel defines neither a size nor a shape, it only gets this when it is physically displayed, for example. on a screen. You can then use this to calculate the pixel density ( PPI - Pixel Per Inch ) and pixel size. -> pixel calculator 

The number of pixels can be specified both 1-dimensional (in a line) and 2-dimensional (in a rectangle).
The following picture illustrates how the number of pixels affects an image:

Usually the color depth is mentioned in connection with pixels, which indicates the number of possible colors of a single pixel.
The following picture illustrates how the color depth of the pixels affects an image:

Pixels for screens

In screens, pixels are usually displayed with the basic colors red, green and blue (RGB). If you take a greatly enlarged picture of it, you get the following result: