I tried to run C-h r from Emacs, but got error
Info file emacs does not exists
After googling i found the package
emacs24-common-non-dfsg. What is the purpose of this package, and why would I want to install it?
I am Using Ubuntu 14.04.
You can use
apt-cache show <package_name> to get the basic information about that package along with the description.
apt-cache show emacs24-common-non-dfsg
Description-en: GNU Emacs common non-DFSG items, including the core documentation This package includes the core Emacs documentation: the Emacs Info pages, the Emacs Lisp Reference Manual, and the Emacs Lisp Intro. . GNU Emacs is the extensible self-documenting text editor. This package contains the architecture independent infrastructure that is not compliant with the Debian Free Software Guidelines. In particular, this includes some of the GNU Emacs info pages, as they are covered under the GFDL, and specify invariant sections. See http://www.debian.org/vote/2006/vote_001 for more information.
This package is needed for Emacs Manual.
It is a non-free package i.e packages that do not meet the Debian Free Software Guidelines.