I looked on Howstuffworks.com and they have CD players, tape players, MP3s…but no records! How do the grooves hold enough information for a song?
It’s analogue.
The grooves in a vinyl sound-recording disc have wiggles that correspond in shape to the desired waveforms of the sound. You play the disc by placing a ‘stylus’ or ‘needle’ in the groove and rotating the disc. As the disc turns, the stylus passes over the wiggles and vibrates, causing analogus electrical waveforms in the circuit it’s connected to. These electrical waveforms are amplified and sent to speakers, vibrating them and creating the analogous pressure waves in the air, that you hear as sound.
The original versions of the system were completely mechanical. The stylus vibrated a speaker directly.
An inverse process was used to record the discs…
It’s simple. The grooves are cut into a wax master by a needle that vibrates to the sound. The vibrations cut into the wax. Later, a “negative,” (i.e., with ridges instead of grooves) metal version of the disk (called a “mother,” which led to the great little job category of "mother repairer) is created. Then vinyl is pressed between two mothers. When the record is played, the phonograph needle is set into vibration by the groove, and the result is amplified.
Not first, but I got cites!
The record groove is sort of V-shaped. The faces of the groove undulate. As the needle is dragged through the groove, these undulations cause the needle to vibrate. This vibration is amplified to produce sound. In mono records, if memory serves, the undulations were normally left-to right, as opposed to up-and-down (feel free to correct me on this). Some early record players, though (Pathe Vertical Cut) were crated for vertical undulations. Try playing one of those on a standard record player.
For stereo records, each face of the groove is cut differently to correspond to the two channels. The resulting 3D motion of the needle is decoded to produce stereo sound.
Maybe you should have tried “howstuffusedtowork.com.” 
Mainly dittos to those who have posted in reply. Sunspace, that’s close, but not quite right. In the early phonographs, there was no loudspeaker, as we know them today. Instead, the stylus (needle) was physically attached to a diaphragm, usually made of thin brass or other metal; the sound waves created by the diaphragm’s manipulation were “amplified” through a “horn,” much like a megaphone. My dad still has a 1916-model Edison phonograph of this configuration. The volume control still cracks me up – it’s a fuzzy “pom-pom” looking thing on the end of a coaxial cable. To lower the volume, one slides the control and the cable moves to stuff the pom-pom farther down the throat of the horn. Pretty clever, actually…
zut, the motion of a stereo stylus isn’t exactly “3D.” A stereo stylus is actually two styli mounted close together but able to move separately. Each half is then able to move independently according to the “bumps” on one side of the groove.
I remember saving my nickels and dimes to be able to afford the premium “elliptical” styli available at the height of hi-fi stereo, pre-CD. Supposedly, the curved sides of the styli fit better into the groove of a record, corresponding more closely to the shape of the ridges of the “mother.”
You sure? I dug up this after a quarter-hour of searching (A better cite than the previous! It’s got pictures!). The stereo stylus pic refers to a “stylus, which moves both laterally and vertically”. The second cite above agrees: “In response to the two undulating sides of the impressed groove, the stylus has both lateral motion (right-left) and vertical motion (up-down). The resultant motion of the stylus is decoded by the cartridge into the two stereo signals.” Are there perhaps different stereo stylus technologies, one with multiple needles and one with a single needle? Or are you thinking of quadrophonic sound? Or am I off base, and if so, where?
Er, I didn’t say an “electromagnetic loudspeaker as we know them today”. 
The horn-and-diaphragm combo together qualifies as a “speaker”, doesn’t it?
Stereo styluses are indeed singles and not doubles.
Actually, the “lateral” vs. “vertical” statement is a bit of a simplification. They wanted to be sure that stereophonic records would play acceptably on monophonic equipment which responded only to lateral motion, and monophonic records would play acceptably on stereo equipment. If one of the stereo channels was lateral, the other vertical, a monophonic playback would only give you one channel instead of a mix of both. Also, playback of monophonic records on a stereo would then only come out in one channel. Essentially, they solved it by recording with the two axes at 45 degrees to the record surface:
I can readily accept how this would work for single tones…but how can one needle, assumably passing one groove at a time, pick up a symphony of different sounds?
The complex horizontal and vertical movement of the needle is exactly analogous to the complex compression and rarefaction of the air surrounding the symphony. A single needle moving in one dimension can do that much, and moving in two dimensions is what gives us stereo. Both sound waves and phonograph needle displacement can be broken down into combinations of pure sine waves of different wavelengths.
There are dual-needle cartridges mentioned by others here, but they do not (necessarily) produce stereo; they are designed so that 78 rpm records and 33 rpm records can be played without changing cartridges
Needles?! Stylus?! Pah! We don’t need that prehistoric stuff!
Play vinyl the modern, digital way! Scan your vinyl then process the image!
Disclaimer: I think the jury is still out on this one. May be a hoax. Amusing though.
What is cool about playing a record is it requires no electricity. You can make a paper cone and attach a sewing needle to the end of it and place the needle on the record while rotating it by hand. You can hear the music from the paper cone…well sort of.
The old grammophone, just crank and listen.
Futile Gesture, I posted that same link in the other vinyl-records thread here in GQ. Great monds think alike!
(Unless that should be “Fools seldom differ.”)
“monds”??? Definitely the line about fools!
Interestingly and somewhat relevantly, there were efforts to use finely-focused beams of light (not lasers), in combination with photoelectric sensors, to read standard-type vinyl records without the wear and tear caused by needle-contact. Not sure if it ever caught on anywhere, though.