There are two "tracks" cut into a stereophonic record, corresponding to the two channels in a stereo signal. The groove for channel 1 is cut into the depth of the record and it makes the needle move up and down. The second is cut into the walls of the record groove and forces the needle to move from side to side. The record stylus is the little piece of diamond coated metal that rides in the groove. It is mounted on a small metal shaft. This tiny shaft sits in a rubber ring and continues into the cartridge. In the cartridge are two small magnetic coils. Each coil has two wires that come out of it. These correspond to the four pins you see on the back of a cartridge.
No power ever comes to the tone arm assembly. It is independent of the rest of the electronics in the turntable and can be replaced at will. The whole sound generation part of playing a record starts out by creating current off the record with the needle and cartridge. Think of your cartridge as a small generator.
This is why you get sound even if the motor of the turntable is off and you move the record. You are not generating power by spinning the record.
Unfortunately this whole process
of reproducing audio using a stylus on a piece of vinyl isn't
as sonically accurate as everyone would like it to be. The signal
actually has to be modified to properly be pressed as a groove
in the record. And when you play the record back it gets modified
again by as special circuit in your phono pre-amp. To sound like
the original recording. This is pretty much the same idea as using
dolby noise reduction when recording and playing back a tape.
Keep in mind the actual process is very different. This process
is called RIAA equalisation
Ironically what most people consider to be warmth in a record is usually due to defects in the lower frequencies. Most engineers agree that there is always a marging for low end error when mixing down a track for vinyl. Although who's gonna complain about a little extra bass..?
different things affect the quality of the signal on vinyl. The type of vinyl is a big one to whatch out for. In dance music it's no secrets that the best pressings are usually German, British and French. These countries have huge markets and great local technology in the music industry. They comonly press using the purest vinyl and press deeper into thicker vinyl. This technique gives a louder and better signal.