We know that batteries store energy - electrical energy. What happens inside a battery when it gives out electricity is actually a chemical reaction. Chemical energy is converted to electrical energy. Similarly the little 'dynamo' attached to a bike converts mechanical energy into electrical energy. Water coming down the Hoover dam converts potential energy to electrical energy by turning the generators. We also use the term 'electromotive force' to refer to this electrical energy (although this is not a 'force'!).
All electrical gadgets in order to work nseed a source of electromotive force - for e.g. a battery which we represent by the symbol
This source of emf is hooked up to a load say a lamp which has a resiatance R to form a circuit - see fig. below.