The simplest hydraulic device that you find in common use today is the log splitter. It contains all of the basic components of a hydraulic machine:

An engine, normally a small four-stroke gasoline engine, provides the power for the system. The engine is attached to a hydraulic oil pump.
The hydraulic oil pump creates a stream of high-pressure oil, which runs to a valve.
The valve lets the operator actuate the hydraulic cylinder to split a log.
There is also a tank to hold the hydraulic oil that feeds the pump and usually a filter to keep the oil clean.

The major components of a log splitter are shown below:

In cross section, the splitter's important hydraulic parts look like this:

High-pressure oil from the pump is shown in light blue, and low-pressure oil returning to the tank is shown in yellow.
Click the button to activate the piston!

In the figure above you can see how the valve can apply both forward and backward pressure to the piston. The valve used here, by the way, is referred to as a "spool valve" because of its resemblance to a spool from a spool of thread.

Let's look at some of the specifics of these components to see how a real hydraulic system works. If you take a trip down to your local building supply center or a place like Northern Tool and Equipment and look at the log splitters, you will find that a typical backyard log splitter has:

A 5-horsepower gasoline engine
A two-stage hydraulic oil pump rated at a maximum of 11 gallons per minute (3 gpm at 2,500 psi)
A 4-inch-diameter, 24-inch-long hydraulic cylinder
A rated splitting force of 20 tons
A 3.5-gallon hydraulic oil tank

A two-stage pump is an ingenious time-saver. The pump actually contains two pumping sections and an internal pressure-sensing valve that cuts over between the two. One section of the pump generates the maximum gpm flow rate at a lower pressure. It is used, for example, to draw the piston back out of a log after the log has been split. Drawing the piston back into the cylinder takes very little force and you want it to happen quickly, so you want the highest possible flow rate at low pressure. When pushing the piston into a log, however, you want the highest possible pressure in order to generate the maximum splitting force. The flow rate isn't a big concern, so the pump switches to its "high pressure, lower volume" stage to split the log.

We'll look at hydraulic pumps in the next section.