This is a breakout board for Honeywell's HMC5883L, a 3-axis digital compass. Communication with the HMC5883L is simple and all done through an I2C interface. There is no on-board regulator, so a regulated voltage of 2.16-3.6VDC should be supplied.
The breakout board includes the HMC5883L sensor and all filtering capacitors as shown. The power and 2-wire interface pins are all broken out to a 0.1" pitch header.