A credit card has a magnetic strip how data is stored on a magnetic strip?

http://www.topbits.com/how-a-magnetic-card-reader-works.html

According to existing standards, a magnetic card stores information in three separate tracks. All three tracks possess different bit densities and encoded character sets. The average bit density of the first track is 210 bits per inch (bpi). A 64-bit character dataset is used to store information in track 1. The characters are made up of six data bits and an odd parity bit. The encoding format grants the least-significant bit to come first and the parity bit, last. So, track one can hold around 79 characters.

The information density of the second and third tracks is around 75 and 210 bpi respectively. Only numeric data can be stored on the second and third track. 4-bit binary encoding is used as a decimal subset with odd parity and it is encoded with the least significant bit first and the parity bit last. The second and third tracks hold 40 and 107 characters respectively. The actual usable data stored will be minimized as the Start Sentinel, End Sentinel, and LRC characters are added.

"Two-Frequency, Coherent Phase Recording" is used for encoding magnetic cards. It is also called as F/2F sampling-encoding. By using combined data and clock bits self-clocking is achieved.

For all 3 tracks, the data storage format is as follows: Leading zero bits are encoded to specify the presence of an encoded magnetic card and provide synchronization pulses to the read head electronics and, ultimately, to the controller. The Start Sentinel character then indicates the initiation of actual data. The coded data follows the Start Sentinel character which is followed by the End Sentinel character. The End Sentinel character terminates the data portion of the card and is followed by an LRC byte (used for error detection). The remaining card is filled by zero bits.