At the time of decimalisation in 1971, the United Kingdom Crown coin (equal to 5 shillings) was redenominated as a 25p coin and pieces struck prior to 1990 - all the way back to 1818 - continue to be legal tender for that amount. The old denomination had been a regular circulating coin in the 19th century, but in the 20th century it became more of a commemorative denomination.
The Royal Mint continued issuing commemorative coins in the same format - using the traditional coin dimensions, but in copper-nickel (75% copper, 25% nickel) just as the last pre-decimal crowns had been. These were initially also denominated as 25 pence but - uniquely for British decimal coinage, did not have their value stated on them, which continues the tradition of the old crowns.
Only four designs were issued between 1972 and 1981. After that, the "Crown" was revived in 1990 as a commemorative Five Pound coin, having the same dimensions and weight but with a face value of £5 - twenty times as great. The two can be distinguished because the £5 coin is marked with its value. Crowns were issued in Brilliant Uncirculated and in Proof FDC grade until 2008, after which they ceased to be struck to a circulating standard and are now only issued as proofs.
The denomination is used to mark special occasions, usually royal in theme, rather than for use in general circulation. Although not circulating, these coins are legal tender, but contrary to popular belief this does not mean that banks and retailers automatically have to accept them. In the United Kingdom, they are accepted by Royal Mail post offices.
Note that the Quintuple Sovereign (5 sovereigns) also has a face value of five pounds, but is a massive gold bullion coin and as such has much higher intrinsic value. | 
