An ambulatory is a semicircular or polygonal aisle enclosing an apse or a straight-ended sanctuary. In the above illustration the ambulatory is represented by the shaded area. A feature of many churchs of Early Christian and Byzantine architecture, the ambulatory was originally used for processions.
An apse is a vaulted semicircular or polygonal termination, usually enclosing a chancel or chapel. A sanctuary is the area around the main altar of a church.
The above discussion is based on descriptions in the Penguin Dictionary of Architecture, Fourth Edition.
Illustration Credit: Wikimedia Commons.