An image in a computer based system is represented by a set of numbers and is therefore called a digital image. Images in the real world are, on the contrary, allways continuous. There is e.g. no way you can specify exactly what color a real-world image is at a specific point. In fact, according to chaos theory you can't even specify the location of a particular point without doing some averaging.
To be able to use an image in a computer we have to collect a number of samples of the analog image. To aquire the needed information we can use an analog camera, e.g. a video camcorder. The camera captures an image and changes the visual information to a stream of continuously changing electrical voltage levels.
The electrical signal that comes out of the camera is continuous in time and it can, ruffly speaking, have any output level within a specified range. This kind of signal is said to be analog.
