Man
To find out more about a command use ‘man’ and then the command name, entering this will give you a program manual for that particular command. E.g.
$ man vim
Once displayed to see the next page, press the space bar or to exit press q at the command prompt.
Manuals come in a number of sections, so to shortcut them use:
$ man 1 passwd
This will jump to section 1 or use
$ man N intro
Where N is the section number you are interested in.
Man Command