A protocol means the rules that are applicable for a network.
Protocol defines standardized formats for the data packets, techniques for detecting and correcting errors and so on.
You can say that a protocol is basically a formal description of message formats and the rules that two or more machines must follow to exchange those messages.
To understand the concept of communication protocol, suppose that there are two persons, PersonA and PersonB. Now let's assume that PersonA and PersonB need to talk to one another to exchange their ideas. But it turns out that, both, PersonA and PersonB are egoists. They start talking again simultaneously, then pause for breath simultaneously, and then start talking again. Now just image the confusion and chaos. To avoid it, they must follow a set of rules while talking to one another. For instance, say first PersonA must talk, then he/she must give PersonB a chance to put forward his/her ideas, and so on. this common set of rules would be known as communication protocol for PersonA and PersonB.
We need protocols every time when we want to do something to another computer.
Every time when we want to print something on a network printer, we need protocols.
Every time when we want to download a file we need again protocols.
Every time when we want to save our work on disk, we do not need protocols, unless the disk is on a network file server.
Generally, multiple protocols will be in use simultaneously. For one thing, computers generally do several things at once, and often for several people at once. Therefore, most protocols support multitasking.
Single operation can also involve several protocols. For example, consider the NFS, stands for Network file System, protocol. A write to a file is performed with an NFS operation, that uses another protocol to perform a function call on a remote host, that uses another protocol to deliver a datagram to a port on a remote host, that uses another protocol to deliver a datagram on an Ethernet, and so on.
Here are the list of some most important and common protocols, you will learn in separate tutorials:
You will learn all about the above communication protocols one by one divided into separate tutorials.