From Envirowiki

A gravity battery is a way of storing energy by lifting a substance vertically a certain distance, storing it, then releasing it, and re-capturing the energy on the way down. On the way up, some form of energy is converted into gravitational potential energy, and on the way down, it is re-converted into a more useful form of energy.

The most common form of large gravity battery is dual-dam system, where electricity is used to pump water from a dam (or other water source) at the bottom of a hill, to a dam near the top of a hill. When electricity is needed, the water is released, and flows down to the bottom of the hill, and the kinetic energy of the moving water is converted to electricity via a turbine (hydro-electricity). The only losses in this system are the efficiency of the pump and turbine, evaporation, and friction (minimal with water).

[edit] uses

Gravity batteries can potentially store large amounts of energy - depending on the holding capacity of the top storage system. They can be used to even out loads in dispersed-network systems prone to intermittance, such as wind power grids, but are probably better at dealing with predictable, short period peak loads.