Several different coins have born the name crown throughout history, but the most famous crown coins are those of Britain, its colonies and territories. The value of these coins depends on the type of crown coin you have.
The earliest British crown coins were made during the reign of Henry VIII around the year 1526. A British crown coin was worth 5 shillings and they were worth a quarter of a pound/sovereign. In addition to British crowns made in Britain, many of Britain's territories and colonies also made crown coins. Britain still occasionally mints commemorative crown and crown-type coins.
British crowns were made predominantly out of gold and silver. Because so many different types of crown coins were minted across such a vast amount of space, weight and design was not uniform.
Modern crown coins began being minted in 1818. Some crown coins from the later 20th century are worth as little as 60p, but crown coins from earlier in the century and the 1800s are often quite valuable (a 1937 Edward the VIII coin with a picture of George IV on it being worth more than £65,000 in 2010).
Older Crown coins were hammered, not minted, so very few survive. The value of these coins (depending on condition, date minted, place of origin and composition) can be quite high, so it would be a good idea to take an older coin to a dealer to see what they know about it before selling it.