Magnetoresistance is an effect observed in (ferro)magnetic
conductors.
When applying an external magnetic field to the conductor, the
resistance of the conductor changes.
The resistance is minimized when the magnetic field is aligned
parallel to the conductor, and maximized when the magnetic field is
aligned perpendicular to the conductor.
This effect was first observed by W. Thomson in 1856, and
changes in resistance are usually below 1% at room temperature and
higher at lower temperatures <5K
For further reading:
http://en.wikipedia.org/wiki/Magnetoresistance
http://en.wikipedia.org/wiki/Electron_scattering