The software world
Operating system
OS
The program that runs your computer and every other program on it — macOS, Windows, Linux. It owns the screen, the disk, the keyboard and the network, and hands out access to them: when you double-click a file, the OS is what decides which app opens it.
Why it matters
Part of Files, folders and paths → It's the reason the same idea has three different costumes. Paths, home folders and where apps live differ between macOS, Windows and Linux — the concepts don't. Knowing which one you're on also tells you whether Claude Code Desktop is available to you at all: it ships for macOS and Windows, and in beta for Ubuntu 22.04+ and Debian 12+, so only the other Linux distributions still mean the terminal.