The easiest way to get more information about a particular Linux command or program is to use the man command followed by the item you want information on:

This will bring up the manual page ("man page") for the program within the shell, which have been formatted from the online man pages. These pages can be referenced from any Linux or Unix shell where man is installed, which is most systems. Linux includes a built-in manual for nearly all commands, so these should be your go-to reference.

The manual is divided into a number of sections by type of topic, for example:

If you specify a specific section when you issue the command, only that section of the manual will be displayed. For example, man 2 mkdir will display the Section 2 man page for the mkdir command. Section 1 for any command is displayed by default.

If your terminal does not support scrolling with the mouse, you can navigate the man pages by using the up and down arrow keys to scroll up and down or by using the enter key to advance a line and the space bar to advance a page. Use the q key to quit out of the manual.

The man pages follow a common layout. Within a man page, sections may include the following topics:

NAME

a one-line description of what it does.

SYNOPSIS

basic syntax for the command line.

DESCRIPTION

describes the program's functionalities.

OPTIONS

lists command line options that are available for this program.

EXAMPLES

examples of some of the options available.

SEE ALSO

list of related commands.

Example snippets from the man page for the rm (Remove) command:

$ man rm
RM(1)                            User Commands                           RM(1)

NAME
    rm - remove files or directories

SYNOPSIS
    rm [OPTION]... FILE...

DESCRIPTION
    This  man page documents the GNU version of rm.  rm removes each 
    specified file. By default, it does not remove directories.

     If the -I or --interactive=once option is given,  and  there  are  more
     than  three  files  or  the  -r,  -R, or --recursive are given, then rm
     prompts the user for whether to proceed with the entire operation.   If
     the response is not affirmative, the entire command is aborted.

Depending on the command, the OPTIONS section can be quite lengthy:

OPTIONS
    Remove (unlink) the FILE(s).

       -f, --force
              ignore nonexistent files, never prompt

       -i     prompt before every removal

       -r, -R, --recursive
              remove directories and their contents recursively

       -v, --verbose
              explain what is being done

Fun fact: there is even a manual entry for the man command. Try:

$ man man

Issuing the man command with the -k option will print the short man page descriptions for any pages that match the command. For example, if you are wondering if there is a manual entry for the who command:

$ man -k who

Since there is a man page listed, you can then display the man page for the who command with man who.