Bash / Shell

This one is a command line "interpreter" for a set of basic commands (ls, cd, cp, etc.) and installed commands/tools (gcc, vim, ssh, etc.).

Files are organized into a tree structure and are specified by their "path" (e.g. /home/thiry or /usr/bin/bash). A path can represent both a file, a directory or any system's resource (eg. process, communication pipe, etc.).

The fundamental command is then help that presents the basic commands.
Next, commands can be organized into categories to manage files, users, processes, network, etc.