The MP3 Format
If you have read How CDs Work, then you know something about how CDs store music. A CD stores a song as digital information. The data on a CD uses an uncompressed, high-resolution format. Here's what happens when a CD is created:
Music is sampled 44,100 times per second. The samples are 2 bytes (16 bits) long.
Separate samples are taken for the left and right speakers in a stereo system.
So a CD stores a huge number of bits for each second of music:

44,100 samples/second * 16 bits/sample * 2 channels = 1,411,200 bits per second

Let's break that down: 1.4 million bits per second equals 176,000 bytes per second. If an average song is three minutes long, then the average song on a CD consumes about 32 million bytes of space. That's a lot of space for one song, and it's especially large when you consider that over a 56K modem, it would take close to two hours to download that one song.

The MP3 format is a compression system for music. The MP3 format helps reduce the number of bytes in a song without hurting the quality of the song's sound. The goal of the MP3 format is to compress a CD-quality song by a factor of 10 to 14 without noticably affecting the CD-quality sound. With MP3, a 32-megabyte (MB) song on a CD compresses down to about 3 MB. This lets you download a song in minutes rather than hours, and store hundreds of songs on your computer's hard disk without taking up that much space.

 Is it possible to compress a song without hurting its quality? We use compression algorithms for images all the time. For example, a GIF file is a compressed image. So is a JPG file. We create Zip files to compress text. So we are familiar with compression algorithms for images and words and we know they work. To make a good compression algorithm for sound, a technique called perceptual noise shaping is used. It is "perceptual" partly because the MP3 format uses characteristics of the human ear to design the compression algorithm. For example:

There are certain sounds that the human ear cannot hear.
There are certain sounds that the human ear hears much better than others.
If there are two sounds playing simultaneously, we hear the louder one but cannot hear the softer one.
Using facts like these, certain parts of a song can be eliminated without significantly hurting the quality of the song for the listener. Compressing the rest of the song with well-known compression techniques shrinks the song considerably -- by a factor of 10 at least. (If you would like to learn more about the specific compression algorithms, see the links at the end this article.) When you are done creating an MP3 file, what you have is a "near CD quality" song. The MP3 version of the song does not sound exactly the same as the original CD song because some of it has been removed, but it's very close.

From this description, you can see that MP3 is nothing magical. It is simply a file format that compresses a song into a smaller size so it is easier to move around on the Internet and store.

Using the MP3 Format
Knowing about the MP3 format isn't half as interesting as using it. The MP3 movement -- consisting of the MP3 format and the Web's ability to advertise and distribute MP3 files -- has done several things for music:
It has made it easy for anyone to distribute music at nearly no cost (or for free).
It has made it easy for anyone to find music and access it instantly.
It has taught people a great deal about manipulating sound on a computer.

Technology has made it easier to download and play your favorite music.
 
That third one was accidental but important. A big part of the MP3 movement is the fact that it has brought an incredible array of powerful tools to desktop computers and given people a reason to learn how they work. Because of these tools, it is now extremely easy for you to:

Download an MP3 file from a Web site and play it
Rip a song from a music CD and play it directly or encode it as an MP3 file
Record a song yourself, convert it to an MP3 file and make it available to the world
Convert MP3 files into CD files and create your own audio CDs from MP3 files on the Web
Rip songs off of various music CDs and recombine them into your own custom CDs
Store hundreds of MP3 files on data CDs
Load MP3 files into tiny portable players and listen to them wherever you go
To do all of these amazing things, all you need is a computer with a sound card and speakers, an Internet connection, a CD-R drive to create CDs and an MP3 player. If you simply want to download MP3 files from the Web and listen to them, then all you need is a computer with a sound card and speakers and an Internet connection -- things you probably already have!
Let's look at many of the different things you can do with MP3 files and the software that makes it possible.

Downloading and Listening

If you would like to download and then listen to MP3 files on your computer, then you need:
A computer
A sound card and speakers for the computer (If your computer has speakers, it has a sound card.)
An Internet connection (If you are browsing the Web to read this article, then you have an Internet connection and it is working fine.)
An MP3 player (a software application you can download from the Web in 10 minutes)
If you have recently purchased a new computer, chances are it already has software that can play MP3 files installed on its hard disk. The easiest way to find out if you already have an MP3 player installed is to download an MP3 file and try to double-click on it. If it plays, you are set. If not, you need to download a player, which is very easy to do.