A tracking shot is where the camera actually moves forward or backward.  This movement brings the audience into the action, almost like they were following, leading or walking alongside the action. Because the movement of the camera creates movement from the perspective of the audience, it lures them into participating or being a part of the scene, which is why it is effective in horror. It effectively drags the audience into something horrific that they actually would want to get away from.

Trucking left or right accomplishes the same thing as above, but rather the camera moves sideways in order to keep the subject in the frame, or it reveals something else in the scene. When you read about camera movement, sometimes they refer to this movement as 'Tracking Left' or 'Tracking Right' - an extension of the above, and sometimes they call it 'Crabbing Left/Right' because the camera moves sideways like a crab.