It is a robot made of Arduino and Lego NXT components based on the Segway.
The gyroscope and accelerometer are combined with complementary filters to provide an inertial reference sensor. The accelerometer provides accurate static tilt information, when the robot is not accelerating. The gyroscope can be integrated to provide accurate dynamic tilt information, but the integration tends to drift over time. Combining the two sensors provides a robust inertial measurement.
A software library to control LEGO NXT motors and a few sensors he used is available on SourceForge. This robot does a good job of balancing itself while moving forwards and backwards.
More detail here.