For more information, see: http://en.wikipedia.org/wiki/Caldwell_catalogue
Sir Patrick Moore was well known in the UK as the presenter of the monthly BBC astronomy programme “The Sky at Night” from April 1957 through to just before his death in January 2013. This 55 year stint represents quite a record. He was also a prolific author and public speaker.
Patrick Caldwell-Moore, to use his full name, felt there were many more interesting objects in the night sky than just the 109 Messier objects so he drew up his own list of 109 deep sky objects (DSOs) that he particularly enjoyed.
Unlike the Messier objects, he chose DSOs from the Southern hemisphere as well as the Northern. To avoid confusion with the M-designation of the Messier objects, Patrick Moore used his middle name, Caldwell, to designate the list.
The list was first published in Sky and Telescope in December 1995.
Most people (myself included) find the Caldwell objects more technically demanding than the Messier objects. Some of them are fuzzy, faint or difficult to see from a light polluted area so filters that reduce Sodium or Mercury light and improve contrast such as UHC (Ultra High Contrast) or Oxygen III emission filters can be helpful.