TCP (Transmission Control Protocol): Reliable, ordered delivery. Sets up a connection first (3-way handshake: SYN → SYN-ACK → ACK). Retransmits lost packets. Used for web browsing, email, file transfer. UDP (User Datagram Protocol): Unreliable but fast. No connection setup, no retransmission. Used for real-time applications where speed matters more than perfection — voice calls, video streaming, online gaming.