Just to follow up (for those who care)... here are some other electric/water analogies that help out.
Voltage vs. Current - A sprayer on a garden hose has a lot of voltage... fast-moving, but not much pressure behind it. A river, on the other hand, has a lot of current; lots of pressure behind it but not a lot of speed of flow. This works with changes, too.... decrease the current (by running it over shoals), and you increase the voltage (the speed at which it moves) and vice-versa. Getting sprayed with a water hose won't kill you... lots of voltage, no current. Falling into a deep river can drag you under; lots of current, no voltage.
A transistor can be compared to a lock on a dam. You've got all that potential current/voltage sitting behind the gate. Something you couldn't hold back by yourself, certainly. But, using the levers or worm gears necessary to open the gate, you let that water out. So... a transistor takes a smaller control signal (operation of the gate) to allow the main current through.
A resistor is, of course, just a restriction in the hose/pipe/whatever that water is flowing through. The more resistance (the smaller the opening), the less voltage and/or current can get through.
A diode simply allows voltage to go one direction, and not another. A backflow valve.
As with water, voltage and current are interchangeable. If you have something that draws 10 ampres of current at 12 volts (it takes 10 units of "push" at a certain pressure to work), then it will take 5 ampres of current at 24 volts. And vice-versa. It would take 20 ampres of current at 6 volts. Think about it; if I have a hose and a sewer pipe, and I need to fill my swimming pool, which will fill it faster? More imporantly, if I slow down the speed of flow (voltage) in my sewer pipe, but have a lot of speed in my garden hose, *then* which will fill it faster?
This is why higher current loads in electricity need heavier guage wire. The wire, like the sewer pipe, needs to be able to handle the additional pressure without bursting.
OK... I'm done for now.. Hope you enjoyed the read.
Just my 2¢
"In order to start solving a problem, one must first identify its owner." --Me
--Greg