Types of ventilation
Passive (natural) ventilation means that you have openings that air flows through with no help from you or the power grid -- just the natural action of wind and the tendency of warm air to rise. Passive ventilation includes an open window, a ventilation slot, a louvered gable-end vent, that sort of thing.
Passive ventilation is the easiest, cheapest, safest, and most foolproof method for the vast majority of backyard coops, in my opinion. Build lots of it.