In the contexts of geometric direction, navigation, missile guidance, etc., bearing is a superset of azimuth. Assuming two objects are on a common plane, the bearing from one to the other may be absolute or compass, defining the north-south-east-west direction of a line drawn from the starting object to the target.
A relative bearing assumes the starting object's "front" (i.e., the bow of a seagoing vessel, the muzzle of a gun, or the nose of an aircraft) is at zero degrees, or some other measurement of angular relationship. Assuming the degree system, the relative bearing is measured clockwise from the zero position, so a target at the "three o'clock" or right angle position would be at relative bearing 090.
If the "front" is traveling due north, the relative and compass bearins will be identical. If, however, a ship were traveling due east (i.e., compass bearing 090), the relative bearing to a ship at "three o'clock" would be 0 degrees.