During upgrades, files with these extensions appear in /etc and its subdirectories. I gather that *.dpkg-dist contains the file that was distributed with the currently-installed version of a package, and *.dpkg-new contains the version from the version being installed, however I'd like to see the docs to be sure that I'm getting it right. Also there are occasionally other similarly named files, eg *.dpkg-original, and I'd like to be able to read up on these.
I've checked /usr/share/doc/dpkg for documentation on this, and come up empty; there's no dpkg-doc package; Google doesn't have anything except unanswered questions. Can someone point me to the documentation for this aspect of debian package management?
dpkg-old and dpkg-dist are filename extensions that are created when a package is upgraded and a conffile overwritten. .dpkg-old is created when the user has chosen to use the newly installed one. .dpkg-dist is created when the user opted to keep the old one; it is the version from the newer package.
Some more information on dpkg's configuration file handling can be found at the Debian Policy Manual, Appendix E - Configuration file handling.
There is also dpkg's log file, /var/log/dpkg.log, which records changed conffiles, and the performed action (keep vs. install).
Source: http://help.ubuntu.com/community/Configuration