Answer 1

I recommend reading about Unix FHS. It defines most standard directories.

Some basics
/bin and /sbin: essential binaries - those the system depends on to boot, reach a working state and do essential tasks
/usr/bin and /usr/sbin: non-essential binaries - well, the other ones
/bin and /usr/bin - binaries executable by regular users
/sbin and /usr/sbin - binaries reserved to root
/usr/local/...: Locally compiled binaries

Some distros, like the RedHat family, have /usr/{,s}bin symlinked to {,s}bin, making the paths interchangeable.