Hydraulic systems are used in all kinds of places. For example, the hatchback of your car probably has hydraulic cylinders to help it open, close, and stay open. And of course, heavy construction equipment uses hydraulic systems in order to do their work.
These systems work using fluids. The fluid can be water, but in most cases, it's an oil or a special hydraulic fluid. Those things can last longer without degrading, which lets the system work longer. The system itself is made up of a series of tubes, cylinders, pistons, reservoirs, seals, and switches. Each part of the system is necessary, and without it, the system won't work.
These things all actually work in the same basic manner. When the fluid is pushed into the system, it is carried through the tube or cylinder, and it is activated. That activation means that the system starts to do what it is supposed to do. For example, the fluid can move into a cylinder, activating it, which causes it to move the shovel of the excavator.
The piston in a hydraulic system can help to push the fluid from one place to the other. You might see a piston in a hydraulic cylinder that opens a building door or a car door.
The seals are really important because they are used to make sure that all the connections are going to hold the fluid in the system. Hydraulic seals are generally made out of some kind of rubber or silicone. They are placed in between the connectors. The seals will help to fill up any gaps in the connections, which would let the fluid run out. The seals need to have some flexibility so that they hold up no matter what happens with the connection. The connection may have to move, and a flexible seal will move with the connection and keep it tight.
The reservoirs hold the hydraulic fluid so that it can be moved through the system. Larger systems are likely to have multiple reservoirs because the fluid is going to need to be moved through several parts of the system.
The switches are used to change where the fluid is and to direct it so that it can activate the system.
Hydraulic systems are used in all kinds of places for all kinds of things. The systems are basically simple, no matter how big and complicated the setup ends up being.