Basic directory structure and layout of a Linux system:
/
Also know as "slash" or the root partition
/bin
Common programs, shared by the system, the system administrator and the users.
/boot
Boot files, boot loader (grub), kernels, vmlinuz
/dev
Contains references to system devices, files with special properties.
/etc
Important system and application configuration files.
/home
Home directories for system users.
/lib
Library files, includes files for all kinds of programs needed by the system and the users.
/lost+found
Files that were saved during failures are here.
/mnt
Standard mount point for external file systems.
/media
Mount point for external file systems (on some distros).
/net
Standard mount point for entire remote file systems - nfs.
/opt
Typically contains extra and third party software.
/proc
A virtual file system containing information about system resources.
/root
root users home dir.
/sbin
Programs for use by the system and the system administrator.
/tmp
Temporary space for use by the system, cleaned upon reboot.
/usr
Programs, libraries, documentation etc. for all user-related programs.
/var
Storage for all variable files and temporary files created by users, such as log files, mail queue, print spooler. Web servers, Databases etc.