Debian GNU/Linux is a free, stable operating system built and maintained by volunteers. It emphasizes stability, reliability, and open-source principles. Debian is the foundation for many Linux distributions (Ubuntu, Linux Mint, Proxmox, etc.) and runs everywhere from embedded systems to supercomputers.

On a minimal Debian installation, you get a clean system with apt (package manager), systemd (init system), and a shell. Everything else is installed as needed. Debian prioritizes stability over having the latest versions—packages are tested before release and security updates are provided for years.

$ apt update                    # update package lists
$ apt install package-name      # install a package
$ systemctl status service      # check service status
$ sudo -l                       # check sudo permissions

Debian comes in three release channels: stable (tested, conservative), testing (newer, less tested), and unstable/sid (cutting edge, experimental). Most deployments use stable. Developers often use testing or unstable for newer tools.

1. Filesystem
2. Shell and environment
3. Users and groups
4. Permissions
5. Text editors
6. Package manager
7. Repositories
8. Services and systemd
9. Processes
10. Logging
11. Networking
12. Configuration files
13. SSH
14. Scheduling tasks
15. Disk and filesystems
16. Kernel and modules
17. Locale and timezone
18. Development tools
19. Release cycle
20. Building packages