American vs. European Options
American-style options can be exercised any time before expiration; European-style options can be exercised only at expiration.
Most US equity and ETF options are American-style, so early assignment is possible. Most cash-settled index options (e.g. SPX) are European-style and settle only at expiration.
The exercise style affects early-assignment risk and pricing — American options are worth at least as much as otherwise-identical European ones because of the extra flexibility.
Example. You can exercise an American-style AAPL call on any trading day; a European-style SPX option can only be exercised at its expiration settlement.
FAQ
Are US stock options American or European?
Almost all listed US single-stock and ETF options are American-style; broad-based cash-settled index options are typically European-style.
Why does exercise style matter?
American style introduces early-assignment risk for sellers and gives holders extra flexibility, which is reflected in pricing.