What is "shutter speed"?
The length of time that the camera's shutter is open.
More to come...
Shutter speed measurement
As the name implies, speed is measured in time.
Impact of shutter speed on bulb photgraphy
In bulb photography the shutter speed is the most significant factor both technically and creatively.
For those of you who've already mucked about with bulb photography (intentionally or accidentally) this may sound like the bleeding obvious: the definition of bulb photography is that the lens is left open for a longer than usual length of time.
However, couple this statement to your choice of subject and bulb photography takes on its true creative meaning.
The creativity in bulb photography is often based on the relative amount of time. A photograph of a stationary object (such as a brick) will most likely look much the same if the lens is left open for 1/1000 second to 1,000 seconds!