Options that can be exercised anytime up to their expiration date, are
American Style Options (American Options). In the United States, most options
are of this type. The American Style option provides an investor with a greater
degree of flexibility than a European-style option.
Options which can only be exercised at their expiration, are European Style
options.
A typically used formula to evaluate European options, is the Black-Scholes
or Black model formula. There are no general formulas for American Style
options.