For the most part interacting with the local network---accessing shared files, logging in to compute servers, or communicating with other users---is very simple. Networking software is invoked automatically, and it hides the physical layout of the network from you, giving you the illusion you are using one large computer system. You can access a file simply by giving its name, no matter which machine you are logged in to or where the file resides, or send mail to other users without worrying about which machine they use or are likely to log in to the next time they read their mail.
The Unix file system is a hierarchical collection of directories. In a stand-alone system the file system maps the directory tree onto local disks. In a networked system, however, some of the nodes correspond to files that reside elsewhere in the local network. Figure 2 shows part of a typical Unix file system and how it might be allocated on a combination of local disks and network nodes. The shaded sections of the tree correspond to parts of the file system that reside on other computers. The software used to implement nonlocal files is the network file system, or NFS. Nonlocal files are also called remote files.
Figure 2: Network File System (NFS).