What is a network protocol and why do we need it?

In computer networking, in order for two end devices to communicate in some way, we need rules. These rules are called network protocols. Network protocols are predefined set of rules for effective communication between two end devices, where as different protocols are responsible for variety of operations, therefore these protocols are organized in layers.
There are two different models in computer networking, responsible for the transmission of the packets. The widely used model is OSI or Open Systems Interconnect. It's open standard, therefore everybody can apply it. It's used for network management and you can clearly see where each protocol functions on each layer. It consists of 7 layers.