先来段英文的：
/bin
This directory contains executable programs which are needed in single user mode and to bring the sys‐ tem up or repair it.
/sbin
Like /bin, this directory holds commands needed to boot the system, but which are usually not executed by normal users.
/usr/bin
This is the primary directory for executable programs. Most programs executed by normal users which are not needed for booting or for repairing the system and which are not installed locally should be placed in this directory.
/usr/sbin
This directory contains program binaries for system administration which are not essential for the boot process, for mounting /usr, or for system repair.
/usr/local/bin
Binaries for programs local to the site.
/usr/local/sbin
Locally installed programs for system administration.
看不太懂，没有关系，来个中文版本的：
/bin 存放所有用户皆可用的系统程序，系统启动或者系统修复时可用（在没有挂载 /usr 目录时就可以使用）
/sbin 存放超级用户才能使用的系统程序
/usr/bin 存放所有用户都可用的应用程序
/usr/sbin 存放超级用户才能使用的应用程序
/usr/local/bin 存放所有用户都可用的与本地机器无关的程序
/usr/local/sbin 存放超级用户才能使用的与本地机器无关的程序
搞笑版：
本来这些个目录都在一起的，后来直到有一天：
Unix开发者的机器的硬盘不够了，新加了一块，挂在/usr上；
又TM不够了，再加一块，挂在/usr/local上；
不知怎么，就变成规范了。。。。