I am preparing for a A+ certification exam and I don't understand what are volts and watts? Can anyone explain?

Having just finished two semesters of college physics recently, I recall that Volts are the difference in electrical potential energy between two points. Watts are the work done. Yeah ... clear as mud, huh?!

We are talking about the computer's power supply aren't we? Accordingly, volts are the difference in electrical charge between the point you are measuring and ground (or some other point). Think of it like measuring the "pressure" produced by the power station station that pushes or pulls electrons through the wire. A higher voltage means more pressure applied to the electrons. Similarly, the higher the voltage, the number of electrons traveling through the wire will be greater (assuming resistance is unchanged). The measurement is taken in relationship to another point, which is commonly ground (zero volts).

Watts is the work accomplished, i.e. the energy needed to switch a transistor from one state to another, or to change the state of a memory bit from "0" to "1", or the energy needed to rotate the cooling fans in the chassis... all that is work being done.

When dealing with a power supply, you want to be sure it can handle the voltage (pressure) it is being plugged into. You also want to know if it can handle the combined workload (in watts) needed by all the computer components (CPU, graphics card, motherboard, memory cards, etc). That's why you need to understand the concepts of volts and watts.

So I can say, I am using a laptop and it's plugged in to the charger. I can say, the electricity coming in from the AC adapter through the wire and into the computer are volts? and the power the laptop is actually using are watts?

Close, @techaddict, close.

The number of electrons coming through the wire into the computer per second is electrical current (not volts). More electrons entering per second, the higher the electrical current. Current is analogous to water moving in a stream, i.e. the water current.

The reason the electrons are moving along the wire and not sitting still is they are being pushed or pulled. The force pushing or pulling is the difference in electrical charge (+ or -). This difference is measured in volts. The -5 volt lead/wire from a power supply has a 5 volt difference from ground (0 volts). A -3 volt lead/wire has a 3 volt difference from ground. The -5 volt lead/wire pushes electrons more forcefully than the -3 volt lead/wire.

Watts is the measurement of work done. Those electrons moving through the wire is causing transistors to toggle, memory (bits and bytes) to change states, cooling fans and disk drives to go round and round, ... all this is work. An analogy is the work done by a lawnmower engine versus a 747 jet engine. The jet engine has a lot more horsepower, and hence does a lot more work per second. If both engines are attached to separate cars, the car with the jet engine will travel a whole lot farther (i.e. do more work) per second than the car with small go-cart engine. Back to computers: a server with many disk drives and several memory cards needs a power supply with a higher wattage rating than say a laptop with only a single disk drive and a single memory card. The server does a whole lot more work per second than the laptop.