Historic periods are a way of categorizing and organizing history into distinct time frames. The term 'ERAS' refers to a long period of time, usually lasting hundreds or thousands of years, such as the Stone Age or the Middle AGES. DECADES are shorter periods of time, typically lasting 10 years, such as the 1950s or the 1980s. Lastly, AGES are also long periods of time, but they are more specific than ERAS, such as the Iron Age or the Bronze Age.