help is a
bash built-in, providing help for
bash commands only
man is the traditional form of help for almost every command on your system, and not only, sometimes also configuration files have their own man page. In Linux distros organized in packages, the relative text is in general provided from the same package providing the command. If you have
bash-completion you can take advantage from TAB to see what man pages are available.
info is an alternative system to provide manual pages for commands, based on
GNU emacs. It is provided mainly for
GNU commands and utilities. It don't seem to be widely adopted from others.