Man

The man command on the console is a powerful and quick way to get the finer details about commands, macros and programming interfaces.

The man system is partitioned in 9 sections. You call man like this:

$ man [section] title

where section is optional and if used one of the following sections and title is the command/function/... you are searching for.

The manual sections are traditionally defined as follows:

1. Commands

   Those commands that can be executed by the user from within a shell.

2. System calls

   Those functions which must be performed by the kernel.

3. Library calls

   Most of the libc functions, such as qsort(3))

4. Special files

   Files found in /dev)

5. File formats and conventions

   The format for /etc/passwd and other human-readable files.

6. Games
7. Macro packages and conventions

   A description of the standard file system layout, network protocols, ASCII and other character codes, this man page, and other things.

8. System management commands

   Commands like mount(8), many of which only root can execute.

9. Kernel routines

   This is an obsolete manual section. Once it was thought a good idea to document the Linux kernel here, but in fact very little has been documented, and the documentation that exists is outdated already. There are better sources of information for kernel developers.

Apropos is a utility to inspire you to find the right command.

$ apropos network

this command will search through the manual pages for the word network and list appropriate commands/functions/...

If you are programming kernel modules there are, as mentioned above under 9, not much help to find in the manual pages. You can get inform about the kernel API from here or here