There is a refundable education credit (the American Opportunity Tax Credit - AOTC or AOC), but it's only available to undergrads (and in most cases only the student's parents are eligible for it).
Grad students are eligible for the Lifetime Learning Credit (LLC). It is a nonrefundable credit. That is, it may only be used to offset an actual tax liability. The LLC is 20% of tuition paid, up to $10K. To claim it, on your return, you must not be claimed as a dependent on anyone else's tax return.