What are the key differences between TCP and UDP?

TCP (Transmission Control Protocol) is connection-oriented, reliable (guarantees delivery and order), has flow/congestion control, but has higher overhead. UDP (User Datagram Protocol) is connectionless, unreliable (no guarantee), has lower latency and overhead. Use TCP for: web, email, file transfer where reliability matters. Use UDP for: video streaming, gaming, DNS, VoIP where speed matters and some loss is acceptable.