In simple terms, in a network you often have a central server which contains all a company's files. This serves files out to individual employees at their workstations (or clients). The system allows everyone in the network to see and work on all the company's files simultaneously, rather than having the files scattered around on single computers. The server has to be reliable and have large fast hard drives often with multiple drives for backup or so that the same files can be spread across several drives for speed of access by more than one person at the same time.
In a smaller group -- say just two workers who need to share files -- you would probably set up a peer to peer network of simpler computers where each has access to some of the other's files.