What do the numbers after command and system call names mean in *nix?

Question

I often see number-suffixed command or system call names in man pages or other documentation, for example, splice(2). Although, some publications don't include the numbers, for example, Splice on Wikipedia.

I never found an explanation of what the numbers stand for and don't even know where/how to begin a search.

1. What do these numbers mean?
2. What is the range for these numbers?
3. Why are they suffixed to function and command names?
4. Which communities use those numbers?

Answer 1

These numbers refer to Unix man sections.

1   Commands available to users
2   Unix and C system calls
3   C library routines for C programs
4   Special file names
5   File formats and conventions for files used by Unix
6   Games
7   Word processing packages
8   System administration commands and procedures

You can specify a section number with the man command by just listing the section number:

man 1 somecommand  

would look for somecommand in section 1 of the man pages.

Wikipedia has some additional information on this as does this page on how to use man pages.