In every cartoon, anime or movie/series/programme that is animated 'cels' are used to create the movement of a certain character or object.  When a motion picture is being filmed the film in the  camera takes shots every 1/4 of a second of the moving person/object that is being filmed. The roll of film is then passed over a projector at great speed and the frames help your brain and eyes to create what appears to be a moving image.
  You can make yourself a small animation just to test this out by getting a few sheets of paper. On these sheets of paper draw a small stickman in the corner, then on another sheet in the same place draw another stickman but this time draw him differently. Once you have a collection of little stickmen, place the pieces of paper on top of one another and the flick the corners of the pages quickly, it will appear as if the little stickman is alive and moving.
 Cartoons and animations are also all based on this formula. A 'cel' is basically the equivalent to the piece of paper that the stickman was drawn on. The artists and directors of cartoons draw the characters onto 'cels' and then film the 'cels'. Once again the film is then passed over a projector at great speed to create a moving image (the cartoon).
 Cels are basically transparent sheets of plastic onto which the character or object is then painted onto. The 'cel' is then placed over the top of a pre-painted background. And then filmed. There are 24 frames or 'cels' used for every second of animation. Every 'cel' is numbered according to its part in the sequence of the scene being filmed. For example, the first 'cel' that is to be used in a certain scene would be numbered 'A1', and the next 'cel' would be 'A2' and so on.

Dougas and Gengas 

 Dougas and Gengas are the sketches that are painted by the artists before the 'cel' is painted and placed onto the background ready for filming.