Linux file system hierarchy (FHS)

In short: Linux has only one directory tree that starts at /. The Filesystem Hierarchy Standard (FHS) defines what each folder is for.

In more detail: Drives have no letters like C: but are mounted at a folder.

In detail

Path	Content
/bin, /sbin, /usr/bin	programs (today mostly merged under /usr)
/etc	configuration files
/home	personal files of users
/root	home of the administrator
/var	variable data: logs (/var/log), databases, mail
/tmp	temporary files
/usr	programs, libraries, data (/usr/share)
/opt	additional software
/dev	devices as files (/dev/sda)
/proc, /sys	kernel information (virtual)
/boot	kernel, initramfs, bootloader
/mnt, /media	mounted drives
/lib, /lib64	libraries

• Everything is a file: devices, processes and sockets appear as files.
• Permissions: every file has an owner, group and permissions (rwx).
• Hidden files: names with a dot (.config).

See also: Important packages in the Linux base system, Linux