Usenet was established in 1980, a full decade before regular folks got access to the internet.  It was conceived by Tom Truscott and Jim Elliott.  Usenet most closely resembles forums that you find around the web today and was originally designed for just such a purpose.  Basically, to communicate in text from computer to computer.

Eventually those using it wanted to be able to share files, but Usenet was a text based system.  As such, they got around this by changing the binary file into a text file, posting it to the server, and then changing it back into a binary file on the other end.

When a file is posted to Usenet, it is not immediately available all over the network.  It is first posted to a local server.  Then, other computers that make up Usenet talk to that first server to see what is different.  If one server has different files, they share them so eventually, all computers on the network have the same files, but it takes time for this to happen.

In the end though, there is no centralized server holding all of the files.  Each server holds all of the files for a period of time known as retention.