J. McCoy's answer is wrong.
Tax credits can be refundable, non-refundable, or both under certain circumstances. Here is a review of each.
REFUNDABLE TAX CREDITS: Refundable credits reduce your tax to zero and the remainder is refunded to you. Examples of a refundable credit are the First Time Home-buyers Credit and the Earned Income Credit.
NON-REFUNDABLE TAX CREDIT: Non-refundable credits reduce your tax to zero and stop. The remained will NOT be refunded to you. An example of this credit is the Child Care and Dependant Credit.
HYBRID TAX CREDITS: Some credits can be non-refundable, but refundable in certain circumstances. The Child Tax Credit is a good example. The credit in non-refundable unless income is between certain levels, then is becomes partially or fully refundable as the Additional Child Tax Credit.
Hope this helps.