Ok, I am starting to understand what voltage is. But i'm sort of confused when it comes to negative voltage. I know that voltage is defined as potential energy per unit charge. I understand it as analogous to "pressure". I don't understand what it means to have negative voltage though. How can "pressure" or even energy be negative?
The sign convention for potential (voltage) is positive for a field from a positive charge, and negative for a field from a negative charge. In the case of a circuit, a point on the circuit with negative voltage means the potential is negative compared to a "neutral" ground.
Voltage is a relative quantity. Pressure isn't allways a good analogy for voltage because pressure is an absolute quantity. Consider a 9V battery. The voltage on the +ve terminal is 9V with respect to the -ve terminal. However it's equally true to say that the -ve terminal is at -9V relative to the +ve terminal. You cannot just say that the +ve terminal is at 9V without citing what the reference voltage is. Frequently we are lazy and don't allways bother to cite what the reference is on circuit diagrams. Sometimes we write a small foot note which says "All voltages are measured with respect to...". Consider the circuit below. What is the voltage on Node A relative to node B? What is the voltage on Node A relative to the 0V node? What is the voltage on Node B relative to 0V node? What would you write on the circuit diagram?
A better analogy for voltage using a constant gravity source such as the gravity near the earths surface is height. Gravitational potential reasonably near the surface of the earth equals g h, where g = 9.80665 m / s^2 and h = height in meters. Treating g as constant, the gravitational potential is a function of height. So if an object is above the surface it has positive gravitational potential, and if an object were below the surface such as inside a hole it would have negative gravitational potential. You could also define gravitational potential to be zero at the top of a 100 meter tower, so that gravitational potential equals g (h - 100). This would result in negative potential for h < 100 and positive potential for h > 100. The units for gravitational potential is joules / kg, while the units for electrical potential (voltage) is joules / coulomb.
As CWatters was pointing out, ground is not always the "least potential" in a dual power supply. The other way to think about it is if you see something like "+5V, Ground and -5V", it's the same thing as "+10V, +5V, 0V" as far as potentials.
Volts and Force / Pressure are risky to use as analogies - for a start, they do not have the same units. Force is a vector and Potential Difference (Volts) has no direction, even if it has a sign. If you think of the definition of Electrical Potential Difference (Voltage): The energy needed to take a unit positive charge from one place to another. Going from an arbitrary 'ground' or 'earth' point to a positive potential involves putting a Positive amount of energy into the system. Going from ground to a negative potential will involve getting work out or Negative Energy needed. This is a good demonstration of how a poor analogy can lead you into worse problems than just using the basic definitions. Potential Difference is only the description of the relationship between the Potentials at two different points. If there are is no 'Earth' label on either terminal of a battery, you will measure Plus Volts or Minus Volts, depending on which way round you connect your meter; it's all relative.