Hydraulic cylinders are used for creating mechanical force in a linear motion. A hydraulic cylinder is a tube capped at either end with a rod sticking out of one side. Attached to the rod, interior to the cylinder, is a piston. The piston separates the internal rod side from the internal cap side of the cylinder. Fluid is forced into either side of the cylinder to extend or retract the piston rod.
The piston rod is attached to the part of the machine requiring motion; it could be the boom arm on an excavator or platen on a press. Literally any application requiring linear application of force is an excellent use of a hydraulic cylinder, and no other method of linear motion is so strong and efficient as a cylinder is. Hydraulic cylinders can extend with force ranging from a couple thousand pounds up to thousands of tons.
Hydraulic cylinders are used in mobile applications, such as excavators, dump trucks, loaders, graders, back hoes and dozers, which is an incomplete list to say the least. They can push, pull and lift loads of any description, and the mobile machinery industry relies nearly exclusively on hydraulic cylinders for linear motion.
Hydraulic cylinders are also used prolifically in the industrial machinery industry. The power density of hydraulic cylinders is unmatched, making them great for presses, compactors, injection molding, forging presses, et al. Even for advances applications such as flight simulators or fatigue testing, hydraulic cylinders can be used in literally any linear motion application.