Sue, if their federal stafford/direct loans, you have a 6 month grace period AFTER you GRADUATE from school before you have to start paying them back. People going into residencies are able to defer because of hardship or other reasons.
So, no you do not have to pay back your loans while you're still working toward your degree, but after you graduate. If you graduate and decide to pursue an advanced degree, then you don't have to pay back your loans until after you finish that advanced degree. For private loans, you'll have to check on their conditions though, this info is only valid for the federal loans or stafford loans.