Pumped-storage hydropower (or PSH) uses water as a mode of energy storage, and the type of energy stored is gravitational potential energy. The way it is done is that there are two reservoirs of water placed at different heights or elevations; the water at the higher elevation has more gravitational potential energy, and so when that water is allowed to rush down to the lower reservoir, that entire gravitational potential energy turns into kinetic energy. That is what eventually drives the turbines, allowing electrical energy to be formed for usage.
As far as storage goes, this can be done by reversing this process! We can accomplish that by driving water up from the lower to higher reservoir, with the use of a water pump. This converts kinetic energy back to gravitational potential energy, which is then stored safely at the higher reservoir for later use.
