A crown is a type of headdress worn by a monarch. They are usually worn when they are crowned or at important occasions. Also, crowns are now used as a symbol of the monarchy. They are also won by beauty pageant winners.
"Crown" is also a synonym for prize or reward. Crowns are awarded or won at contests. Beauty contests award crowns to the winning participant, for example. Couples chosen the King and Queen at a dance also receive crowns.
Some children, mainly girls, connect flowers in a chain and wear them on their heads as if they were crowns. These are called daisy chains and have nothing at all to do with crowns. A crown can also be a part of the head.
A diadem is a type of crown, an ornamental headband worn by monarchs and others as a badge of royalty.