The Leyden Jar is an early type of capacitor, invented in the 18th century, that stores electrical energy. It consists of a glass jar lined with metal foil on the inside and outside. When charged, it can hold a significant amount of electrical energy, which can be released all at once. Named after the city of Leyden in the Netherlands, where it was first developed, the Leyden Jar played a crucial role in the study of electricity. It was used by scientists like Benjamin Franklin to conduct experiments, helping to advance the understanding of electrical phenomena.