Boring:
Enlarging a hole by means of a single-point cutting tool. Boring may be done to obtain a continuous inside diameter, or a stepped, tapered, contoured or recessed diameter. Boring may be performed on either a lathe or a drilling machine.

Counterboring:
Enlarging a portion of a hole. Counterboring may be performed on either a lathe or a drilling machine.

Countersinking
Process of making a cone shaped enlargement at the entrance of a hole. Countersinkng may be performed on either a lathe or a drilling machine.

Drilling:
Making a hole in a workpiece where none previously exsisted. Drilling may be performed on either a lathe or a drilling machine.

Facing:
Process of making a flat surface across the face of a workpiece. The faced surface (usually an end of the workpiece) is at right angles to the lathe axis and the part itself. Facing is performed on a lathe.

Reaming:
Enlarging a hole to accurate size. Reaming must be preceeded by a drilling or boring operation. Reaming may be performed on either a lathe or a drilling machine.

Tapping:
Process of cutting internal threads. Tapping may be performed on either a lathe or a drilling machine.

Threading:
Making a thread on a workpiece (such as a screw). Thread cutting can produce either inside or outside threads that are either straight or tapered. Threading is performed on a lathe.

Turning:
Removal of material from the outside diameter of a workpiece to form a cylidrical surface. The surface may be straight (one continuous diameter), tapered or contoured (as a concentric but irregularly shaped surface). Turning is performed on a lathe.

Conventional Milling:
Also called "Up Milling". The direction of the motion of the milling cutter tooth as it engages the work is opposite from the direction of the movement of the work caused by the table feed. Because of this the table and the workpiece will never have a tendency to pull towards the workpiece because of loss motion between the nut and the table screw.

Climb Milling:
Also called "Down Milling". The Milling Cutter and the workpiece move in the same direction. The velocity of the cutters teeth is greater than the velocity of the table feed, which moves the work into the cutter, producing the chip. This pulls the workpiece into the cutter by the action of the cutting forces. This can damage the workpiece, cutter, and the Machine.

More information to http://www.physics.harvard.edu/machineshop/machineshop.htm