Museums started out as private collections. Collectors would keep all kinds of fascinating or odd items in special cabinets that were made to display things. These cabinets were referred to as "Cabinets of Curiosities". (Now you know why we use the phrase "curio cabinet"!) 

These collections of oddities and special treasures would be acquired by others. As the collections grew and were added to, they became rather large and took up entire rooms rather than a simple cabinet. Eventually they were made public and became museums!