Variation and deviation both effect a magnetic compass. One is predictable based on th elocation and is annotated on hte compass rose of HO charts. The other is induced by locaL masses of iron or steel on board and is correct by having an adjuster swing the compass.

Variation:the angle (at a particular location) between magnetic north and true north
www.cogsci.princeton.edu/cgi-bin/webwn

deviation -- (the error of a compass due to local magnetic disturbances)
Ibid