You're lying in bed, drifting off to sleep. Just as your thoughts become a whirl of images, and your body drifts into a lesser consciousness, you imagine yourself riding a bicycle, or on a roller coaster, or running down a hill. In your mind you stumble and fall and your body actually feels like it's falling! You jerk awake and fall into your mattress, without ever really moving. You pull it together and wonder, "What in the world just happened?"
You experienced a myoclonic (or hypnic) jerk. At this stage of sleep, you're still easily awakened, but your body is preparing for deep REM, during which muscles aren't "allowed" to move. During transition, the larger muscles of your body might contract suddenly, causing you to wake up. That you happened to be dreaming about falling is, in this scenario, a coincidence. Other folks believe your muscles contract because of your dream, as they're not yet restricted from movement by the brain. Whatever the reason, it is this mild jerking motion which startles you, and it's not anything to worry about.
In fact, I find the feeling quite pleasant (if not a little scary) and usually spend the next 10-15 minutes trying to do it again.
You can read more about it at discovery.com