Shutter

The shutter physically moves out of the way to allow light to reach you film or sensor. It's a very delicate mechanism, capable of opening and closing in as little as 1 1000th of a second. A good shutter consists of 2 parts, known as curtains, that move across the sensor in the same direction, making sure that each portion of the sensor is exposed for the same length of time. A bad shutter, like the one on my 1980's Praktica, just has one part, that moves off the film and then back, thus ensuring that one end of the frame gets more light than the other.