A UNIX computer system is called a host because it can allow many users to share its resources. Digital UNIX organizes information into files and directories.
Files are individual documents.
The file system organizes the relationships between files and directories, much as a filing cabinet organizes papers and folders. The location of each file and directory is its path. A pathname is a full description of where a file or directory is stored in the file system. Consider a forest full of trees, where each tree is similar to a UNIX host. If a hostname lets you pick out a particular tree, then a path points to a specific leaf on that tree.