While you're waiting for the appraisal to be completed and the report recieved, your lender is underwriting the rest of the file behind the scenes.
As Marine said, after this you should receive a "conditional" loan approval. The lender will provide you with a list of any outstanding conditions that the underwriter would like to see before issuing a clear to close for the loan.
Common conditions are: proof from your bank that any earnest money deposits have cleared your account, verification of your employment from your HR department, proof of adequate homeowner's insurance, etc. If the underwriter sees anything that seems unclear or fishy, then they are going to add a condition to address it.
Depending on the lender you are working with, conditional approval can take anywhere from a couple days to several weeks. Some are better than others at turning this around quicker, and some won't even review things until the appraisal is back and they have a complete file.
Bottom line, make sure your loan officer is keeping in touch with you, communicating, and providing regular updates whether or not they have any new news to report.