Periods in history are divided into three categories: EPOCHS, ERAS, and AGES. An epoch is a period of time that is usually longer than an era and is marked by significant changes or developments in a particular area. An era is a period of time that is usually shorter than an epoch and is marked by a particular set of characteristics or events. An age is a period of time that is usually shorter than an era and is marked by a particular set of characteristics or events. All three terms refer to distinct periods in history and can be used to describe different periods of time.