Update: For some history and the real reason why there is /usr, read this: http://lists.busybox.net/pipermail/busybox/2010-December/074114.html. Also this.
/sbin - Binaries needed for booting, low-level system repair, or maintenance (run level 1 or S)
/bin - Binaries needed for normal/standard system functioning at any run level.
/usr/bin - Application/distribution binaries meant to be accessed by locally logged in users
/usr/sbin - Application/distribution binaries that support or configure stuff in /sbin.
/usr/share/bin - Application/distribution binaries or scripts meant to be accesed via the web, i.e. Apache web applications
*local* - Binaries not part of a distribution; locally compiled or manually installed. There's usually never a /local/bin but always a /usr/local/bin and /usr/local/share/bin.