Step 1: Contrast the two structures:
Where the gopuram is an outward-facing monumental gateway announcing the temple's presence, the mandapa is an inward, functional hall for worshippers.
Step 2: Describe the gopuram's form:
Gopurams are tall, tapering, richly carved towers, a hallmark of Dravidian temple architecture, especially prominent in Tamil Nadu temples.
Step 3: Describe the mandap's form and use:
Mandapas are pillared halls used for assembly, rituals, and cultural performances, situated between the entrance and the garbhagriha.
Final Answer:
\[ \boxed{\text{Gopuram: gateway tower; Mandapa: pillared assembly hall}} \]