This is a basic RFID tag used for presence sensing, etc. Works in the 125kHz RF range. These tags come with a unique 32-bit ID and are not re-programmable. Card is blank on one side, smooth, and mildly flexible.
EM4001 ISO based RFID IC
32-bit unique ID
64-bit data stream [Header+ID+Data+Parity]
Dimensions: 2.13 x 3.35 x 0.03" (54 x 85.5 x 0.8mm)