/usr: The Big One

The /usr directory is the main application-storage directory. The binary files in this directory are not required for system start-up or maintenance, so the /usr hierarchy can be, and often is, located on a separate file system. Because of its (usually) large size, /usr has its own hierarchy of subdirectories. We will mention just a few:

Let's also mention the /usr/share/doc and /usr/share/man directories, which respectively contain applications' documentation and the system's manual pages.