Online documentation

The majority of the online documentation is available through the man command. There are three important ways you can use man.
man -k keyword
gives you the table of contents line for every man page that contains the keyword in the table of contents line. This can sometimes help you locate something when you don't know what it is called.
man title
gives the first man page for that title. For example: 
man man
would give you the page describing how the man command works. Almost every command on the system has a page describing it.
man -s # title
gives you the man page in a specific chapter (#) about title. For example, kill is in several chapters: 
      # man -k kill
      kill (1)		- terminate a process with extreme prejudice
      kill (2V)		- send signal to a process 
      kill (3F)		- send a signal to a process 
      killpg (2)	- send signal to a process group 
      xkill (1)         - kill a client by its X resource
If you just said 'man kill', you would probably get kill from chapter 1. To specify kill from section 2, you would type: 
      man -s 2 kill

Manual organization

The manuals are broken up into eight chapters, as follows:

Chapters 1 and 8 contain commands that you can type at the command prompt. Chapter 6 describes various games available in the directory /usr/games. (Please restrict game playing to after hours, between 8pm and 8am.)

Chapters 2 and 3 describe functions that you can call from your programs. Chapter 5 describes the formats of special files in the system.

Each chapter also has its own subsections.

For example: 

      kill (1)		- terminate a process with extreme prejudice
      kill (2)		- send signal to a process 
      kill (3F)		- send a signal to a process
kill (1) is a command you can type from the shell prompt.
kill (2) is a system call available to C programs.
kill (3F) is a fortran 77 library subroutine call.
The following commands would give a manual for each: 

      man 1 kill
      man 2 kill
      man 3f kill
Also, each chapter has an intro page that lists subsections, and contains other useful information. 
   intro (1)	- introduction to commands
   intro (2)	- introduction to system services and error numbers
   intro (3)	- introduction to user-level library functions
   intro (3F)	- introduction to FORTRAN library functions
   intro (3L)	- introduction to the lightweight process library (LWP)
   intro (3M)	- introduction to mathematical library functions and constants
   intro (3R)	- introduction to RPC service library functions and protocols
   intro (4)	- introduction to device drivers, protocols, and network interfaces
   intro (5)	- file formats used or read by various programs
   intro (6)	- introduction to games and demos
   intro (7)	- miscellaneous useful information pages
   intro (8)	- introduction to system maintenance and operation commands

Also, some tutorials are available via answerbook.