The Internet is a World Wide network of computers connected together using internationally defined protocol's 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.

So a Dos/Windows machine, an Apple Macintosh, or UNIX machine can communicate as they all can use the common language.