Magnetic stripe card

Magstripe (magnetic-stripe) cards are everywhere: bank cards, hotel cards, gift cards, transit ticket, 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.

Encoding of data varies, possibilites include:
 * ISO/IEC_7813
 * ANSI/ISO ALPHA Data Format
 * ANSI/ISO BCD Data Format

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)

More sources

 * Wikipedia page has good details
 * Magnetic Strip Encoding Standards