TCP, UDP and what a port is for

Two transports

An IP address gets you to a machine. A port gets you to a program on it.

TCP

Connection-oriented. It performs a handshake, numbers every segment, retransmits what goes missing and delivers bytes in order. Anything where correctness matters more than latency uses it: HTTP, SSH, database protocols.

UDP

Fire and forget. No handshake, no retransmission, no ordering. That sounds worse until you are carrying live audio, where a packet arriving late is worth less than nothing.

Ports worth memorising