An apse refers to a semicircular or polygonal recess at the end of the nave in a basilica, typically housing an altar.
A basilica is a large rectangular building with multiple sections including an apse. It serves as both public gathering places and religious spaces.
Altar: An altar is usually found within an apse and serves as the focal point for religious rituals and ceremonies.
Chancel Arch: The chancel arch separates the main body of a church from its sanctuary or chancel area which often includes an apse.