The compact GY-521 module is based on the MPU-6050 NEMS chip, and combines a 3-axis gyroscope and a 3-axis accelerometer with a digital motion processor (DMP) and a temperature sensor.
The sensor data is read out via the I²C bus, therefore you only need 2 lines (SCL and SDA) besides ser power supply. Via pin AD0 you can set the I²C address (Low = 0x68,High = 0x69).
The chip has a 1024 byte FIFO buffer, which can be read out e.g. by a microcontroller. A sub-I²C bus allows the connection of further I²C sensors, e.g. for the determination of air pressure, compass direction, or similar.
Thanks to the integrated "Digital Motion Processor" (DMP), complex calculations can also be executed within the chip to relieve the microcontroller.