this should actually be below /usr/lib for consistency, but the tradition is overwhelmingly in support for this name
/usr/lib
contains precompiled libraries for use by everyone on the system. for instance, in /usr/lib you can find the file libstdc++.so.6, which is needed for C++ programs. (similar to the folder containing all the .dll files in Windows)
contains unchanging data files for programs and subsystems, including some site-wide configuration files. The name lib comes from library; originally libraries of programming subroutines were stored in /usr/lib
contains libraries for the binaries in /usr/bin and /usr/sbin
/usr/lib<qual>
alternative format libraries, e.g./usr/lib32 for 32-bit libraries on a 64-bit machine (optional)
/usr/share/man
/usr/share/info
/usr/share/doc
/usr/share/man - manual pages
/usr/share/info - GNU info documents
/usr/share/doc - miscellaneous other documents
/usr/bin
contains executables for common UNIX utilities
contains non-essential command binaries (not needed in single user mode); for all users
/usr/local
The place for locally installed software and other files. Distributions may not install anything in here. It is reserved solely for the use of the local administrator. This way he can be absolutely certain that no updates or upgrades to his distribution will overwrite any extra software he has installed locally
tertiary-hierarchy for local data, specific to this host
typically has further subdirectories (e.g., bin, lib, share)