To expand on
retrolefty's explanation of "voltage differential", in a simplified "layman's understanding", which is more than sufficient for an explanation, current "flows" from "high voltage" to "low voltage".
The low voltage is called "ground" (for historical and litteral reasons), and is typically 0 volts (but it doesn't have to be!); so 5V will flow to ground, or 0V, because there is a difference of 5 volts.
If your ground level were instead 2.5 volts, it would still flow, because there is a difference of 2.5 volts. If the ground level were -5 volts, it would flow because the difference is 10 volts.
There is such a thing called a "dual-ended" supply, one which "outputs" a positive and negative voltage; you sometimes see components (operational amplifiers, especially) that need such a supply. It is made by using a dual-ended supply with a common ground.
Imagine if you hooked up two 9V batteries in series, so that you had an 18 volt potential across their two end terminals. If instead, you tapped off the center connection between the two batteries, and called that "ground", you would now have a "dual-ended" supply, where one side output +9 volts, and the other side -9 volts (of course, on the negative side of the supply current is actually flowing from ground to -9 volts - something to keep in mind when you design such circuits).
These are the basics. If you want the real skinny on all of this (and I suggest you probably should read up on it) - find a copy of Grob's "Basic Electronics"; its well worth studying.
