This is a 5V, 10A 2-Channel Relay interface board. It can be used to control various appliances, and other equipments with large current. It can be controlled directly with 3.3V or 5V logic signals from a microcontroller (Arduino, 8051, AVR, PIC, DSP, ARM, ARM, MSP430, TTL logic).
It has a 1×3 (2.54mm pitch) pin header for connecting power (5V and 0V), and for controlling the relay. The pins are marked on the PCB: