Most likely it is a brushed DC permanent magnet motor, though some treadmills use series-wound "universal" motors that are either AC or DC--not permanent magnet, but using field coils instead. A very few that I've seen use AC-only motors with startup caps.
They are not usually very lightweight, nor small. Take a look on my Electricle blog for the treadmill motor posts for a few pics of the one I bench-tested on an early version of CrazyBike2's frame, using 3 car batteries in series. May also be some pics in my early page or two of my CrazyBike2 thread here on ES, and also pics of another larger heavier type in there from about two years ago? Maybe three. It's still in a treadmill in the pics.
Thing I liked at the time about treadmill motors is they are supposed to handle their rated power for a long time (all day, AFAIK) instead of being rated for bursts of power. But they often require fan cooling. Typically that's done by blades on the back of the flywheel/belt drive they often use to drive the big rollers for the treadmill belt itself that you walk on. On mine, I intended to overload it, more or less, so I figured out a way to put a car alternator's fan on there. There's better ways to do it and better fans, but I had one and it fit so I used it.
But since treadmill motors are high RPM, and need high voltage to get there, you end up with a more complicated system than using a low RPM motor that runs on low voltage...like a powerchair motor, whcih is what I eventually used for CB2 for that reason (and that it had a right-angle gearbox on it, which GREATLY simplified mounting!).
Someday I still want to use a treadmill motor to power a bike, essentially teh same mechanical way I started to with CB2.
