Question: What is protocol?
Protocol is agreement between computer devices on how to communicate between each other. This is the standards that vendors will follow when they manufacture computer devices, so that the devices from different vendors could talk to each other. This is also the standards for software house to use when they write computer program.
Some examples: When you type http://google.com in your web browser, the browser will talk to Google web server by using HTTP protocol, and your FTP client will talk to FTP server by using FTP protocol in order to send/receive files.
The popular protocols are IP, TCP, UDP, HTTP, HTTPS, FTP, SMTP, POP3, IMAP, etc.