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.
A diadem is a type of crown, an ornamental headband worn by monarchs and others as a badge of royalty.