The American Opportunity Credit is available for each student for the first $4000 in qualifying expenses for a credit of up to $2500. Of the credit 60% is applied to tax liability and 40% is refundable even if you have no tax liabity. To be qualified the student must be in the first four years of post secondary education and must be at least a half time student.
The Lifetime Learning credit is similar but limited. It is available to any student no matter how many years they have been in school or how many degrees they have, however, it is only for a maximum of $2000 per return, no matter how many students and is 20% of he first $10000 in expenses. This credit only offsets tax and is not refundable.