Welcome to the most crowded folder in the Linux file system. This directory contains the installed programs which are open to all users, the programs’ libraries, header files, manual pages, X Window system, telnet, ftp etc.. According to the Linux standards, /usr directory is shareable, meaning that all users can access the contents but can not modify them.
It is interesting to see that there is also a /sbin folder under /usr. The programs in /usr/sbin are the ones that can be run by the root only for system administration purposes. For example useradd command, which adds users to the system resides in /usr/sbin.
The /usr directory contains a lot of subdirectories which are very important to know one by one, such as /usr/local, /usr/include, /usr/bin, /usr/share, /usr/src. I strongly recommend you to google about these directories and learn about their contents if you want to move ahead with Linux.