A fuel cell is an electrochemical device that converts the chemical energy from a fuel (usually hydrogen) and an oxidant (usually oxygen) directly into electricity, heat, and water through a reaction. Unlike batteries, which store energy, fuel cells continuously produce electricity as long as fuel is supplied. They are used in various applications, from powering vehicles to providing backup power for buildings. Fuel cells are known for their high efficiency and low emissions, making them a promising technology for sustainable energy.