CNC milling is the process of removing material from a solid block of material by using a computer to move a cutting tool on and around the solid block of material which is fixed in place throughout the process unless it needs to be "repositioned".
A 2 axis milling machine can only remove material from the X and Y axis at one time. The X and Y axis are commonly referenced as the length and the width of the part. A 3 axis milling machine adds the additional ability of removing material from the Z axis which is also commonly referenced as the height of the part. A 3 axis milling machine gives you the ability to make much more complex parts than a 2 axis machine.
CNC turning differs from cnc milling in that with cnc turning, the part is turned while the cutting bit remains in a fixed position. In cnc milling, the block of material being made into the part remains in a fixed position while the cutting tool moves around the block of material guided by a computer.