Filesystem layout

What lives under /, /etc, /var, /home, and /usr.

Linux inherited a shared directory tree. You do not install an app into C:\Program Files. You drop pieces into directories that already have a job.

/ — the root of the tree.
/bin and /usr/bin — user commands. On modern distros these are often merged.
/sbin — admin commands.
/etc — configuration.
/var — logs, queues, and other data that changes.
/home — user files.
/tmp — scratch space, wiped on many systems at boot.
/proc and /sys — kernel interfaces that look like files.

ls /
ls /etc
df -h