What exactly is a "bushwacker" in terms of the Civil War?

I have this information and would like to know about what it means and implies:

CRENSHAW, Jesse Sims, Confederate Bushwacker
Born about 1825. Moved from Pickens Co., Ala. to Choctaw Co., Miss in the early 1850's. Descendents say he was a “bushwacker”, and that he was assigned to find deserters. Died 1/20/1908. Buried at New Haven [Miss.?].