Proximity Cards, or "Prox" as they are often called, are standard size plastic ID cards which contain a coil antenna and a pre-programmed micro chip containing a unique code.
When the prox card is within a foot or so of the Prox reader, the 125 KHz signal from the reader is picked up by the card antenna and used to power-up the micro chip which then replies with its own unique code. The reader and its associated processor compares the code with a list of authorized entrants, and if it's OK, the door is opened and a record of entry is logged. Using tomorrow's technology today we have the ability to print directly onto the latest generation of prox cards, or onto peel-and-stick cards which can be applied to the prox card on issue, thereby facilitating reuse of the expensive "prox". Prox cards must always be "personalized" with a portrait ID to eliminate the misuse of "loaned" or stolen cards.