MP3 is short for "MPEG-1 Layer 3," which identifies a way to store digital
audio in a smaller than normal file size.

MPEG (pronounced "EM-peg") is an acronym for "Motion Picture Experts Group," 
which has defined a set of standards for compressing and storing digital audio
and video.  

Regardless of where a sound comes from, what you actually hear is analog 
sound (voltage and current moving speakers which move air to make sound). 
Computers translate and store sound information as digital audio data.
This is done through sampling—the process of taking a digital 'snapshot' 
of the sound many times per second. CDs store information in a digital audio
format known as CD-DA, which samples 44,100 times per second. A Windows Wave
(WAV) file containing "CD Quality" audio is stored at a resolution of
16-bit/44.1 kHz sampling rate. 

MP3 files give you near CD-quality sound in a file format that requires
roughly 1 megabyte for every minute of sound. CDs and CD-Quality WAV files,
by contrast, require about 10.5MB per minute. This means that a single song
or track in MP3 format usually takes up between 3 and 5 megabytes, a
reasonable download even for slower phone modems. Because of this, a profusion
of MP3 Web sites, newsgroups, and FTP sites have sprouted up across the
Internet. It's the MP3 format that has made portable players such as Apple's
iPod possible, and so popular. 

MP3 files are made smaller than WAV files by means of "lossy compression,"
a process which actually discards 'unimportant' audio data. The decisions for
what data to discard are based on psychoacoustics—the science of how the human
brain perceives sound. Studies have determined that not all of the sound we
hear is discernible as separate sounds. 

For example, most people can't hear sounds above a high frequency (pitch) 
of 16 kHz. Also, if a loud sound happens at the same time as a quieter sound, 
and both have a similar pitch (frequency), only the louder sound will be 
perceived by the listener (the loud sound "masks" the quiet sound).  

To create an MP3 file, an MP3 encoder reads a WAV file and then strips out 
the parts that you won't miss hearing. Very high frequency sounds are 
discarded along with quiet sounds masked by louder sounds. 

By whittling away the parts you don't perceive, the MP3 "encoder" creates a
file that sounds almost exactly the same as the original file, but in a file
size that is dramatically smaller than that of the original uncompressed WAV
file or CD-DA data. 

An MP3 file can also contain information about the file itself, in a text
"tag." The tag can contain things like the artist's name, a graphic (usually
the CD cover art), a URL for more information, another URL where you can buy
the CD, the song's lyrics, the genre of music, and more.
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