TCP: connection-oriented, reliable (acknowledgments, retransmission), ordered delivery, 3-way handshake (SYN/SYN-ACK/ACK).
UDP: connectionless, unreliable, fast, low overhead.
TCP = guaranteed delivery (file transfer, email, HTTP). UDP = fast, accept occasional loss (DNS, VoIP, video streaming, games). Security: SYN flood exhausts TCP state table. DNS uses UDP (small queries) but TCP for zone transfers and large responses.