Options can be divided in two categories relatively to the date which can be exercised.

American style options

They can be exercised at whatever date before their expiration or when they expire. Most exchanged traded options are American style (i.e. options in US stocks).

European style options

European style options can be exercised only at their expiration and not before. They are traded on a discount relatively to American options with identical characteristics on the same underline. That’s why the American style options give the extra advantage of early exercise. Most of the European style options are traded over the counter.