filesystem hierarchy standard

Detail of directory names

/opt stands for optional (as in optional add-on packages).
/bin stands for binary (contains executables used by the OS).
/lib stands for library (contains shared libraries used by filesystem and for booting, probably used by the executables in bin)
/proc stands for processes.
/root means root user.
/home holds the home sub-directories for any non-root users.
/dev stands for device (holds special and device files).
/tmp stands for temporary.
/srv stands for serve.
/mnt stands for mount point (mount a temporary filesystem here).
/include contains #include files, i.e. header files (e.g., stdio.h).
/var stands for variable
/etc stands for etcetera. The name is inherited from the earliest Unixes, which is when it became the spot to put config-files.
/usr stands for Unix System Resources(Source) ****
/opt VS /usr/local

Use /usr/local for things that would normally go into /usr, or are overriding things that are already in /usr. Use /opt for things that install all in one directory, or are otherwise special.