info – Read documentation in Info format.
Apart from Man pages, some commands and applications come with a detail manual pages, called info pages, which is in info format. This info command provides Menu, Search, Cross-Referance and many other facilities.
$ info — List all info pages available in the system.
$ pinfo — Curses based lynx-style info browser.
$ info emacs — Show Emacs info page. (See man emacs, to feel the difference)
Read: info info, info pinfo