/usr file system
The/usr file system is often large because all programs are installed here. All files in/usr are typically from Linux distribution, locally installed programs and other things under/usr/local. This may not be necessary to reinstall the entire program when upgrading a new system or a newer distribution.
/usr/x11r6
All files for the X Window System. To simplify the development and installation of x, X's files are not integrated into the system. X himself under/usr/x11r6 similar to/usr.
/usr/x386
Similar to/USR/X11R6, but for X11 release 5.
/usr/bin
Almost all user commands. Some commands are in/bin or/usr/local/bin.
/usr/sbin
Root file systems are unnecessary system administration commands, such as most service programs.
/usr/man,/usr/info,/usr/doc
Man pages, GNU information documents, and various other document files.
/usr/include
C Programming Language header file. For consistency this should actually be under/usr/lib, but it is traditionally supported by this name.
/usr/lib
The unchanging data file of a program or subsystem, including some site-wide configuration files. The name Lib originates from the library; The original inventory of programming is in/usr/lib.
/usr/local
Locally installed software and other files are placed here.
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/var File System
/var includes data to be changed when the system is generally run. Each system is specific, that is, not shared with other computers over the network.
/var/catman
The source file for the Cache.man page of the man page when the format is requested is generally in/usr/man/man*, and some man pages may have preformatted versions that exist in/usr/man/cat*. While the other man pages need to be formatted for the first time, the formatted version exists/ Var/man, so that other people look at the same page without waiting for formatting. (/var/catman are often cleared, just like a temporary directory.)
/var/lib
The file to change when the system is running normally.
/var/local
Variable data for programs installed in/usr/local (that is, programs installed by your system administrator). Note that if necessary, even locally installed programs use other/var directories, such as/var/lock.
/var/lock
Lock the file. Many programs follow a convention that creates a locked file in/var/lock to support their use of a particular device or file. Other programs note this lock file and will not attempt to use this device or file.
/var/log
Log files for various programs, especially login (/var/log/wtmp log all to the system logon and logoff) and syslog (/var/log/messages store all core and System program information.) The documents in/var/log often grow in an uncertain way, should be cleared regularly.
/var/run
An information file about the system that is saved until the next boot. For example,/var/run/utmp contains information for the currently logged-on user.
/var/spool
Mail, news, print queues, and other queues. Each of the different spool has its own subdirectory under/var/spool, for example, the user's mailbox is in/var/spool/mail.
/var/tmp
A temporary file that is larger than/TMP allowed or that requires a longer period of time. (Although the system administrator may not allow/var/tmp to have very old files.)
From: http://www.cnblogs.com/Jamesliang/articles/1486690.html
