Filesystem hierarchy standard (FHS)

/Root-DirectoryMain directory of the entire filesystem hierarchy
/binBinariesEssential commands that need to be available in single-user-mode and should be system-wide (for all users)
/bootBoot loaderBoot-Loader files contain critical information about the system boot process, don’t mess up anything in here!
/devDevice FilesVirtual Filesystem containing interfaces to hardware devices that are present on your computer
/etcEt ceteraConfiguration files for system-wide applications
/homeHome DirectoryUsers home directories containing saved files and user-specific configuration files
LibrariesLibraries that are needed by applications in the /bin folder
/mediaRemovable MediaMount points for removable devices like CDs or USB drives
/mntTemporarily Mounted FilesystemsYou can place manually mount points for storage devices, etc. in here
/optOptional FolderThird-Party vendor applications or self-compiled/developed applications
/procProcessesVirtual Filesystem containing information about system processes
/runRun-time variable DataTempfs Filesystem running in the RAM, containing temporary data of applications
/sbinSystem BinariesSimiliar to /bin, containing binaries that only a superuser should execute
/srvServer DataSite-specific data like data on Web- or FTP-servers
/sysSystemContains information about kernel features, devices or drivers
/tmpTemporary filesTemporary files stored by applications
/usrUnix system resourcesContains a mix of applications, libraries or shared files between applications and services
/usr/binNon-essential binaries
/usr/includeShared include files
Libraries for binaries in /usr/bin
/usr/localHierarchy for local data (typically contains also /usr/local/bin, /usr/local/lib, …)
/usr/sbinNon-essential system binaries
/usr/shareArchitecture-independent shared data
/usr/srcsource-code (typically kernel sources and headers)
/varVariable DataContains files that expect to grow in size, like logfiles or databases, etc.