In general there are 3 categories of energy storage; Mechanical, Thermal, and Chemical. Storing energy mechanically is commonly achieved by storing water behind a dam until the floodgates are opened, or compressing air or spinning a flywheel. Thermal storage can be hot or cold, ice can be used to store energy as can super heated liquid salt in some solar thermal power generation systems.
A more familiar form of storing electricity is the use of batteries, which utilize a variety of chemicals to hold and release power until needed. Rechargeable battery technologies have been evolving rapidly in recent years, prices have been declining as research continues. Because so much renewable power generation relies on variable inputs, depending on weather or not the sun is shining or the wind is blowing, large scale industrial sized grid connected battery systems are going to become more and more important for our rapidly changing power transmission and distribution systems.