Exercise & Assignment
Exercise is the holder using the option's right; assignment is the OCC selecting a writer (by random allocation) to fulfill the obligation; American-style options exercise any time before expiration, European-style only at expiration.
Most options are closed or expire rather than exercised, but assignment risk is real for in-the-money short positions, especially near ex-dividend dates for calls. Standard equity options expire on the third Friday of the expiration month.