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 digital motion processing ("DMP"), and a temperature sensor. Colloquially called 6 axis gyroscope.
The sensor data are read out via the I2C Bus, therefore you will need in addition to ser power supply only 2 wires (SCL and SDA). On Pin AD0, you can set the I2C address (Low = 0x68,High = 0x69).
The Chip has a 1024-Byte FIFO buffer, which for example can be read out via a microcontroller. A Sub-I2C-Bus allows you to Connect other I2C Sensors, for example. for the determination of air pressure, compass direction, etc.
Thanks to the integrated "Digital Motion Processor" (DMP), you can run complex calculations inside the chip, in order to relieve the microcontroller.