Hydraulic systems are used in many, many places. They have been around for centuries because they are an easy system to create and to use. Even with all that, not everyone knows how hydraulic systems are set up or how they work. 

Liquids

Hydraulic systems work on liquids. In the past, those liquids would've been water, but these days, there are other fluids that work just as well, if not better. Various kinds of oils or other synthetic fluids are what are more commonly used. The fluids used in the system depends on the particular usage of the hydraulic system. A system that is going to get a lot of heavy use is going to need to have a thicker fluid that isn't going to break down easily. If the fluid breaks down, it will cause problems with the system. 

Reservoirs and Valves

The way the system works is that it is made up of a series of reservoirs and valves. As the valves open, the fluid moves from one reservoir to the next. That movement is what makes the system work. For example, as the fluid leaves reservoir A and goes to fill reservoir B, it causes a backhoe arm to lift. When the valve is reversed, the arm will go back down. Other reservoirs and valves will allow other parts of the machine to work in other ways, depending on what they need to do. That could be something like making the bucket move up and down or to dig down into the ground.

Hydraulic systems that complex can have all kinds of reservoirs and valves and all the levers that control all those valves. Then you have the much more simple systems, like the hydraulic tubes that are used in your car's hatch. Those are basically just a tube with a piston and fluid. As the hatch opens, the fluid moves from one area to the next, and then goes back again when the piston goes back down. Not only does that hydraulic cylinder make the car door easier to open, but it also makes it possible to have an automatic open/close feature, and will make sure that the door doesn't fall down or close too suddenly. 

Hydraulic systems are a very basic, very simple system that shows up in all kinds of places. You can contact local services that provide hydraulic seals to learn more about your options.