The shutter speed on a camera is the time for which a shutter is open at a given setting; the length of time a camera shutter is open to expose light into the camera sensor.
How it works
If the shutter speed is fast, it can help to freeze action completely. If the shutter speed is slow, it can create an effect called “motion blur”, where moving objects appear blurred along the direction of the motion.
How it's measured
Shutter speeds are typically measured in fractions of a second, when they are under a second. For example 1/4 means a quarter of a second, while 1/250 means one two-hundred-and-fiftieth of a second or four milliseconds. Most modern DSLRs can handle shutter speeds of up to 1/4000th of a second, while some can handle much higher speeds of 1/8000th of a second and faster. The longest shutter speed on most DSLRs is typically 30 seconds.
To take "ghost pictures" you set the camera up on a tripod and set the shutter speed to a slow setting, such as 10-30 seconds. Stand still in one spot in front of the camera for about half the time and then step out of frame for the remaining half. When you step out of frame the light you were blocking by standing in front of the camera will come into the lens, giving you a transparent effect.
To take "stop motion" pictures you need to be in a well lit area so that enough light is taken into the camera on a fast shutter speed setting, such as 1/4000. The lens of the camera will take the picture faster, or as fast as the motion being made so that it appears as there is no motion in the picture.