Magnetic stripe card

Magstripe (magnetic-stripe) cards are everywhere: bank cards, hotel key cards, gift cards, transit tickets, ID cards. They encode machine-readable data in a strip of magnetized material, usually located next to the long edge of the card. They most commonly appear in the approximate size and shape of a credit-card, though airline boarding passes, mass-transit fare cards, and many other varied forms exist. The underlying technology is the same, though: a sequence of alternately-polarized magnetic stripes which are read with a magnetic head similar to that found in any cassette player.

Data formats

 * ANSI/ISO ALPHA Data Format
 * ANSI/ISO BCD Data Format
 * ISO/IEC_7813
 * TI-59 magnetic card (saves programs from programmable calculator)

Making a reader
Magnetic card readers capable of decoding the data formats used for credit cards are available pretty cheaply on the Internet, but for arbitrary formats you might need the raw data on the card. It's pretty easy to build a reader that uses a soundcard's DAC to capture the signal, by wiring a tape-head from a cassette player to an audio plug, and using a ruler to line up the tape head as you run it along the desired track. (more detailed instructions)

Binary encoding
Modern cards are usually Aiken Biphase (aka F2F).

Python script to decode Aiken Biphase from a WAV file

Count Zero's canonical walkthrough of magstripe technology from Phrack 37

More sources

 * Wikipedia page has good details
 * Magnetic Strip Encoding Standards
 * ALPHA and BCD format info
 * Card standards
 * history of the magstripe card
 * collection of resources
 * Textfiles.com things related to cloning cards

Other links

 * Debuting a universal MetroCard that falls short
 * How Does a Plastic Bag Fix a Buggy Credit Card?
 * The long life and imminent death of the magstripe card