Generally, you will be taxed when investments are sold and in your investment account and you have experienced a gain in any of those investments that are sold. You are not taxed on any gains in your account until those gains become realized meaning the securities are sold. Portions of your investments may be sold when you account is rebalanced in order to keep your strategy on track. If you withdraw funds from your investment account that will require securities to be sold and could generate taxes for any gains you may have realized.
The rate you are taxed at is determined by how long you've held the investments in your account and/or by your income bracket. For further information about your particular tax situation, please contact a tax specialist.