The UART Plug contains a hardware serial port with 64-byte buffers for both sending and receiving. It can support baud rates up to 115200 baud, though this will probably tax the bus through which it is being accessed.
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 two solder jumpers on the board allow up to four UART Plugs to be used together on the same I2C bus.
The Assembled UART Plug has the SMD-sized SC16IS740 serial chip, resonator, and capacitor already soldered onto the board. 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 UART Plug. No parts, no headers.
The documentation and full design details are freely available.