American-style options are contracts that can be bought/sold or exercised anytime before the expiry date. Individual stocks can be purchased or sold at any time.
European-style option contracts cannot be bought/sold or exercised after the expiration date. All Index options can only be exercised or bought/sold on the expiration date.
The Option contract's nomenclature can be used to quickly determine if it is American or European. It is CALL European-style option if CE is found in the contract title. Similar to the above, CA in a contract name signifies CALL American style. PE is PUT European style. PA is PUT American style.