There are four possible outcomes to a review:
Approved. No further action is required and the principal investigator is free to initiate the study.
Revise and Resubmit. Changes, revisions, or additional information is required before the study may begin. These must be resubmitted and the IRB must approve these before the study can begin.
Denied. The proposed research, because of the level of risk involved, cannot be initiated.
Exempt. The project does not require IRB approval or approval is deffered to the IRB of the project host institution.