In a client/ server application there are two major classes of Software:

Client Software:

Client Software usually residing on an end users desktop and providing navigation and display.

Server Software usually residing on a workstation or server class machine and providing backend data access services( where the data can be something simple like file or complex like a relational database). As an example: E-mail- the protocol is simple mail transport protocol and its purpose is allows the transmission of text message and binary attachments across the internet technology.  Another one is Chat and its protocol name is internet relay chat protocol, it provides away by one user to talk to one another in real time over the internet. The real time chat groups are called channels.