A network protocol can be viewed as a common network communication standard, which is used to define a method of exchanging data over a computer network. Network protocols define rules and conventions for communication between different devices, participating in a computer network.
Almost all protocols used in the Internet are common standards defined by RFC (Request for Comments).
Examples of standard network protocols are TCP (Transmission Control Protocol), UDP (User Datagram Protocol), IP (Internet Protocol), ARP (Address Resolution Protocol), HTTP (Hypertext Transfer Protocol), FTP (File Transfer Protocol), TFTP (Trivial File Transfer Protocol), SMTP (Simple Mail Transfer Protocol), SSH (Secure Shell), Telnet etc.
Devices participating in network communication should know about the common network protocol and communicate according to the way, defined by the network protocol. In other words, standard network protocol software must be running on both devices participating in network communication.
To explain it more clearly, if you are using your browser to browse web pages from a web server (example, www.omnisecu.com), you are using a protocol called HTTP (Hypertext Transfer Protocol). Your computer must request web pages from web server using HTTP and the web server must response back to your computer using HTTP.
How HTTP (Hypertext Transfer Protocol) should work is defined as a common standard, RFC (Request for Comments) 2616. Anyone can follow the common standard and create their own Browser or Web server.