This board is almost identical to an Arduino UNO R3, but made in a single sided PCB without SMD components. So, it lacks the chip that comunicates via USB to the computer. Instead of that, it has to communicate serial at TTL levels using an adapter like these ones.
For this purpose it has 2 connectors, a male six pin at 90º with the typical pin assigment (although there is no standard). The other is a RJ12 (6P6C) connector which permits polarized connections, less prone to errors. The board can be powered from this connectors, from the USB +5V line. Also, the board can be reset by the DTR line in the connector, which is used during the programming of Arduino by the standard IDE.
To the computer, the board appears as the "USB to TTL" adapter used. For exmple, in GNU/Linux the board appears as /dev/ttyUSB0 instead as /dev/ttyACM0 like a normal Arduino UNO does.
The board respects the physical design of Arduino UNO R3 (inludes the new I2C pins) and is compatible with all shields, except those requiring a 3.3 V supply.