To make it short: a rectifier is what takes an AC and makes it DC. Hunh?
Check it- so you have an AC wave- looks like a sine wave, it goes up, maxes out, then goes down, and maxes out on the negative side-
basically going positive to negative.
A rectifier diode will totally cut out the negative hump of that sine wave.
So what you would see would be the first hump that goes up, maxes out and comes down, but instead of the second hump (keesp going down, maxes neg, and then comes back up) you just get zero. Then the positive repeats again.
If you make a swanky little bridge of rectifiers, you can flip the negative voltage to positive- so instead of getting a positive hump and a flat line, you get two positive humps.
Then you can add a capacitor to act as a voltage buffer, and instead of humps you almost get a straight line at your maximum voltage level.
Now this is all well and good if you are trying to build some electronics at radio shack, but is that what you want to know?!