lilypad accelerometer

hello

I am currently doing a 2 wheeled balancing cart project and i have a few doubts about the IMU. i have a IMU 5DOF frm sparkfun. it has X, Y, Z, XRATE AND YRATE. On reading a few threads here i have understood that the accelerometer gives me the acceleration and hence the tilt angle can be calculated.

what i dont understand is what do i get from the rate gyro?? all i know is when i connect it to the arduino, the serial monitor shows a voltage! how do i convert this into the angular velocity?? and why do we need to calculate this angular velocity?? i read that the rate gyro id used to calculate the angular velocity (angular rate), but dunno how to get it!!
plz help

thank u in advance