If elkos doesn't answer, according to some research I did, 'reaction wheels' are...

If elkos doesn't answer, according to some research I did, 'reaction wheels' are mostly used for changing attitude, and attitude is measured using either star tracking or sunray angle measurement.

One source: http://bluecanyontech.com/portfolio-posts/cubesatsystems/

Note: I'm not particularly familiar with CubeSat development other than being very curious about it.