Program Manager is the person who manages one or more Programs in an Organization or its PMO. Since, Programs are comprised of projects, the Program Manager will set the direction of execution and track the progress of all the constituent projects. As a result, the Program Manager may have other Project Managers reporting to her.
In general, a Program Manager may be looked upon as a position higher up in the hierarchy as compared to a Project Manager. However, that is not always the case. For instance, I could be a Program Manager and be managing a Program worth $2 Million, while you may be a Project Manager and be managing a Project $150 Million. The example above considers only project budget for relative comparison, while there may be other parameters to compare projects and projects.