Step 1: Define Gopuram:
A Gopuram is the monumental, pyramid-shaped entrance tower of a South Indian (Dravidian-style) temple, marking the boundary/gateway of the temple complex, often heavily adorned with sculpted figures of deities.
Step 2: Explain Gopuram's role:
Over time gopurams grew taller and more elaborate than the temple's central shrine tower (vimana) itself, becoming the visually dominant feature of temple complexes such as at Madurai and Srirangam.
Step 3: Define Mandap:
A Mandapa is a pillared hall found in temple architecture, positioned in front of the main sanctum (garbhagriha), used for the gathering of devotees, ceremonies, dance and music performances.
Step 4: Explain Mandap's role:
It served as a transitional, congregational space linking the outer world to the sacred inner sanctum, and temples often had multiple mandapas for different ritual functions.
Final Answer:
\[ \boxed{\text{Gopuram = temple gateway tower; Mandap = pillared congregational hall before the sanctum}} \]