Carbon brushes deliver power to the rotating part of the motor - they rest against a segmented cylinder on the rotor of the motor called a commutator. As each segment of the commutator passes under the brush it's quite normal for a small spark to jump across the gap.
You can see a home made electric motor sparking away in this video...
http://www.youtube.com/watch?v=MxA3pQlpu84
Peccavi, September 2009
