Hands-on Guide for Linux /usr file and folders

  /usr/bin/X11 -> /usr/X11R6/bin
  /usr/lib/X11 -> /usr/X11R6/lib/X11
  /usr/include/X11 -> /usr/X11R6/include/X11

man1: User programs
Manual pages that describe publicly accessible commands are contained 
in this chapter. Most program documentation that a user will need to 
use is located here.

man2: System calls
This section describes all of the system calls (requests for the kernel 
to perform operations).

man3: Library functions and subroutines 
Section 3 describes program library routines that are not direct calls 
to kernel services. This and chapter 2 are only really of interest to
programmers.

man4: Special files
Section 4 describes the special files, related driver functions, and 
networking support available in the system. Typically, this includes 
the device files found in /dev and the kernel interface to networking 
protocol support.

man5: File formats
The formats for many data files are documented in the section 5. This 
includes various include files, program output files, and system files.

man6: Games
This chapter documents games, demos, and generally trivial programs.
Different people have various notions about how essential this is.

man7: Miscellaneous Manual pages that are difficult to classify are 
designated as being section 7. The troff and other text processing 
macro packages are found here.

man8: System administration Programs used by system administrators
for system operation and maintenance are documented here. Some of
these programs are also occasionally useful for normal users.