How Access Card Works:

Access Card or Security passcards are often used to gain entry into areas and buildings with restricted access. The security passcard may be for general access, meaning that the passcard does not provide data about the person using it, or it may be individually encoded, containing specific information about the cardholder.
Typically, the data on an encoded security passcard includes:
Name
ID # (social security number or other unique number)
Access level (where you're allowed to go)
An individually-encoded passcard looks a lot like a credit card. The stripe on the back of the security passcard is a magnetic stripe, often called a magstripe. The magstripe is made up of tiny, iron-based magnetic particles in a plastic-like film. Each particle is really a very tiny bar magnet, about 20 millionths of an inch long. The magstripe on the back of the card is very similar to a piece of cassette tape.
The magstripe can be "written" because the tiny bar magnets can be magnetized in either a north- or south-pole direction. When the polarity of the bars aligns in the same direction, the card is blank. Writing the data requires the use of an encoder. A change in strength can reverse the polarity of the tiny magnetic bars in the magstripe if they are positioned in the gap of the encoding solenoid. By reversing polarities in a certain sequence, the encoder writes data to the card.
A card reader can understand the information on the magstripe. A reader detects the changes in its magnetic field caused by the flux reversals on the passcard's magstripe. Most readers use one of three methods for reading the card:
Swipe reader - You swipe the card through a long, narrow slot that is open at each end.
Insert reader - You insert the card into a small receptacle that is just large enough to accommodate the card.
Proximity reader - You hold the card in front of the blank face of the reader.