[rfc791.ORG]
The first manual page ever written for unix was said to cover the `cat' utility, and was said to have been written by this guy here, Dennis Ritchie, who happens to also be inventor of the C programming language.

Dennis Ritchie passed away this past weekend, and will be missed by a couple generations of hackers who have appreciated having his shoulders to stand upon.

[ rfc791.ORG : Unix Help : The Unix Manual ]

man - The Unix manual
The Unix manual is embodied in a program called `man'. To get help on almost any command, you can type:

man <name of command>
For example, to learn more about the `kill' command, you could type:

man kill
The flaw in `man' is that you need to know the name of the command you want help on before you ask for it. Actually this is only partially true - you can look up a word in a manual page description using the `-k' parameter: 

man -k kill
Also of use to know: the manual has several numbered sections as follows (usually):
  1. General commands / Tools and utilities
  2. System calls and error numbers (for programmers)
  3. C Library reference (for programmers)
  4. Devices and device drivers (for administrators)
  5. Configuration file formats
  6. Games
  7. Miscellaneous documentation
  8. System maintenance and operation (for administrators)
  9. System kernel interface (for kernel programmers)
Usually if someone wants you to look at a man page they will refer to it as follows:

who(1)

This refers to the manual page about the `who' command in the tools and utilities section of the manual. To read this page, you'd type the following to the shell: 
man 1 who
That is unless you're using Solaris (or possibly another SystemV-derived unix) 
man -s 1 who
Usually the man command will give you the first manual entry it finds, so 
man who
Will find who most likely in section 1.