Protocols define how data is transmitted and interpreted. HTTP and HTTPS are for the web, SMTP and IMAP are for email, DNS is for converting names into IP addresses.

On the website, I have compiled brief descriptions of the most popular protocols, their features, default ports, and typical issues. For example, why HTTPS is important not only for security but also for performance (HTTP/2, HTTP/3).

This overview helps to understand how familiar services work and what to do if something stops working. https://tcp-udp-ports.com/