What is difference between under exposed and an over exposed image?

Movie or still Photography film has an emulsion, or exposure speed. This is marked on the box and on 35mm speed still shot camera films. Common sppeds are 100, 200 and 400; the higher the number, the faster the exposure time; cameras are set on tirpods and a shutter cable is used to make the camera steady. High speed film is grainier than slow speed film. Portraits are usually taken, using slow speed film with the camera on a tripod. This number tells you how long the film should be exposed, usually a fraction of a second and if your camera is set correctly, the film will be properly exposed. If a slower setting is used than is correct, then the photo will come out OVER-exposed and will be progressively darker, depending on HOWslow the film was exposed. Makers of film usually include Instruction for proper use in the box or package. New unused films likes cold surroundings better and many people store their unused film in their regrigerators where it keeps much longer. Leave it in the unopened boxes until you actually use it. For asll-around use, 200 speed 35mm color film is the one most-commonly used by less-experienced photographers. Rookies should stay away from films slower than 100 speed; using it requires brains and experience and rookies usually have neither and will ruin a lot of expensive film AND their vacation memories since the film would most likely come out black, after it is developed by professionals. When learning darkroom procedures and developing film, NEVER plunge into it by offering to develop other people's film for the practice and NEVER attempt to develop important pictures for practice either. You'll screw up the film, lose friends and you might just get your butt kicked, or be sued.