The Internet is a World Wide network of computers connected together using internationally defined protocols and standards. Simplistically you can think of a protocol as a language by which computers talk to each other in much the same way as humans use a spoken language such as English to communicate.
As these computers all use the standard protocol they can "understand" each other. This means that they can exchange information and data with each other even if they use different operating systems. Thus a Dos/Windows machine, an Apple Macintosh, or UNIX machine can communicate as they all can use the common language.
As the network grows and expands, so the resources and services available on the network evolve and multiply. A number of tools have been developed to help with the task of locating and retrieving network resources, so that users anywhere can utilise texts, data, software and information for public access. These publicly available facilities can be used to explore software repositories, to consult mailing list archives and databases, to retrieve directory information and to participate in global group discussions.
The key to availability of network resources is the provision of servers on computers all over the network. A server consists of special software which accepts requests (or queries or commands) and sends a response automatically. Requests received by the server may have originated from a user on the same computer as the server software, or from a user on a computer on the other side of the world.
Links have been established between many servers, so that once you have established contact with one server, you can easily communicate with other servers as well.
Software programs which ask for resources from servers are called client programs - they are clients of the server software. Clients send requests to a server, using a standardised format called a protocol. The server responds by supplying information, usually in the form of files containing text or data of various sorts.
New client software is being developed all the time, providing better and more convenient ways of interacting with servers. Different versions of a particular client must developed for different desktop computers. Thus a different version of a client will need to be provided for use for each of the major operating systems.