In the world of computers and networks, a protocol is an agreed-upon set of rules or procedures for transferring data between different devices. When two computers use the same protocol to communicate, it means they’ve agreed to use the same structure for the data they are exchanging, and that they will follow the same steps when they are sending or receiving information.
Since all digital devices (computers, smartphones, tablets, routers, and so on) manipulate numbers (digits), any information they work with must be encoded as numbers. When is encoded on one computer, it must be decoded on the other to communicate. A protocol is essentially an agreed upon way to encode and decode data, and then also set of rules on how to communicate between digital devices. By using the same communication protocols, two very different computers (say, an iPhone and a Windows computer) can exchange data accurately, even if they each encode that data differently.
Think of the postal system: we address envelopes using a set of conventions that make it easier, even possible, to direct the envelope to the correct destination. For example, the address on the middle of the envelope is the destination address, and the address in the upper left (or back) of the envelope is the return address. If there’s an error in the destination address, the postal service uses the return address to send the envelope back to the sender, with “address unknown” stamped over the original destination. This is an example of how postal protocols make it possible to deliver mail and how to handle errors in addresses.
Computers use similar rules (where to put the address and return address, what format to use) and similar procedures (what to do when the address is incorrect, and how to notify the sender) but these networking protocols are much stricter and complex.
Some examples of common Internet Protocols include: TCP/IP, HTTP, SMTP, IMAP and FTP.