If you are a non-US person, you will be notified to submit a Form W8 during the application. This will designate your country of taxation and, if it is not the US, you are not subject to US taxes on your trading profits. Virtually every country, however, imposes withholding taxes on dividends paid by local corporations to non-residents.
The default rate is 30% although it may be lower if an international tax treaty involving your country exists.