Even if human or animal power is sometimes (but highly uncommon) used, usually power tools work on one of the three principles: electrical, hydraulic or pneumatic. The high boost in required parts has also altered the way in which power tools are controlled. Until the 1940s, most industrial manufacturing was based on levers and gears, but soon afterwards, numerical controls (and more recently computer numerical controls) have increased the productivity of these appliances.
However, the latest development in the field of full scale appliances is the machining center, which comprises elements and specifications of individual power tools into one singular machine.
A lot of modern models can replicate the same action over and over again, thus greatly increasing productivity and leaving workers with an administrative role (which is as expected a lot easier that traditional manual labor).
A few examples of very popular machine tools are the drill press, the screw machine, saws, grinding machines etc, but the most common and best known (from before the 1840s) is, of course, the milling machine. There are a lot of distinct models of milling machines, but generally they can be found in one of two alternatives: vertical and horizontal (though they vary in size from a normal workbench to a room sized appliance) and their main functions are cutting, planning, drilling etc.
The major difference between the vertical milling machine and its horizontal counterpart is the position of the spindle axis. The vertical appliance can be lifted or brought down according to need, thus being prefered for heavy industrial manufacturing. However, to choose one or the other implies to know the particular needs of the buyer. Without considering the details of the final work-piece, it can be quite difficult to give an unbiased advice. It is also best to know that while the horizontal machine was highly used before CNC, vertical milling machines are far more popular at the moment.