Ygtjki

The history of these section numbers goes back to the original Unix Programmer's Manual by Thompson and Ritchie in 1971.

The original sections were

1. Commands


2. System calls


3. Subroutines


4. Special files


5. File formats


6. User-maintained programs


7. Miscellaneous

konqueror also describes non-standard sections: (thanks to @greg0ire for the idea)

0 Header files
0p Header files (POSIX)
1 Executable programs or shell commands
1p Executable programs or shell commands (POSIX)
2 System calls (functions provided by the kernel)
3 Library calls (functions within program libraries)
3n Network Functions
3p Perl Modules
4 Special files (usually found in /dev)
5 File formats and conventions eg /etc/passwd
6 Games
7 Miscellaneous (including macro packages and conventions), e.g. man(7), groff(7)
8 System administration commands (usually only for root)
9 Kernel routines
l Local documentation
n New manpages

What it's means already described, but I also wants to add that each section has special manual page with introduction: intro. For example, see man 1 intro or man 3 intro and so on.

From the man manpage:

The table below shows the section numbers of the manual followed by the 
types of pages they contain.

 1 Executable programs or shell commands
 2 System calls (functions provided by the kernel)
 3 Library calls (functions within program libraries)
 4 Special files (usually found in /dev)
 5 File formats and conventions eg /etc/passwd
 6 Games
 7 Miscellaneous (including macro packages and conven‐
 tions), e.g. man(7), groff(7)
 8 System administration commands (usually only for root)
 9 Kernel routines [Non standard]

As to why they're separate like that -- there's some overlap. Certain manpages exist in more than one section depending on what you mean.

For instance, compare man crontab with man 5 crontab -- chances are the latter is the one you meant to look up.

These are section numbers.
Just type man man or open konqueror and type man://man and you'll see what are these sections.

Often, a man page is referenced via suffixing it with the section enclosed in parentheses, e.g.:

read(2)

This style has two main advantages:

• it is immediately clear that you reference a man page - i.e. you can write something like 'cf. read(3)' instead of 'cf. the section 3 man page of read'


• if multiple sections contain man pages with the same name, specifying the section is more precise

Man pages are organized in sections, e.g. Section 1 includes all user command man pages, Section 2 all man pages for the system calls, Section 3 is for library functions etc.

On the command line, if you don't explicitly specify the section you get the first matching man page, in the default section traversal order, e.g.:

$ man read

displays BASH_BUILTINS(1) on Fedora. Where

$ man 2 read

displays the man page for the read() system call.

Note that the positional specification of the section is not portable - e.g. on Solaris you would specify it like this:

$ man -s 2 read

Usually, man man also lists some of the available sections. But not necessarily all. For listing all available sections one may list the subdirectories of all directories listed in the default man path or the environment variable $MANPATH. For example on a Fedora 23 system with some development packages installed /usr/share/man has following subdirectories:

cs es id man0p man2 man3x man5x man7x man9x pt_BR sk zh_CN
da fr it man1 man2x man4 man6 man8 mann pt_PT sv zh_TW
de hr ja man1p man3 man4x man6x man8x pl ro tr
en hu ko man1x man3p man5 man7 man9 pt ru zh

The directories with the man prefix represent each section - while the other ones contain translated sections. Thus, to get a list of non-empty sections one could issue a command like this:

$ find /usr/share/man -type f | sed 's@^.*/man(..*)/.*$@1@' 
 | sort -u | column
0p 1p 3 4 6 8
1 2 3p 5 7

(the sections ending with p are POSIX man pages)

To view a man page in another language (if available) one can set a language related environment variable, e.g.:

$ LC_MESSAGES=de_DE man read

Also, each section should have an introduction man page named intro, e.g. viewable via:

$ man 2 intro

The definitions for SVr4 are:

1 User Commands
2 System Calls
3 library Functions
4 File Formats
5 Standards, Environment and Macros (e.g. man(5))
6 Games and Demos
7 Device and Network Interfaces, Special Files
8 Maintenance Procedures
9 Kernel and Driver entry points and structures

These is the actual numbering for a "genetic" UNIX.
POSIX does not define numbers.