The RTC Plug is a small board containing a low power real-time clock with battery backup, which continues to track the time while external power is switched off. The battery life is good for some 2-3 years on real-time clocks as the battery only keeps the clock chip oscillating when the board is powered down.
An I2C bus running at 3.3V is used as interconnect, with all the power and signal lines brought out to to both sides of the board to allow daisy-chaining with other I2C-conformant boards.
The Assembled RTC Plug comes with the SMD-sized DS1340 chip, crystal, battery holder, and decoupling capacitor already soldered onto the board plus a CR1220 battery. Headers for this and all other JeeLabs plugs can be found here (straight) and here (right angle).
The PCB Only version contains just the printed circuit board for the RTC Plug. No parts, no headers, no battery.