Masonjar is correct. It depends.
Like I know my Soundtraxx Sound/Decoder equipped loco will not run on regular DC.
HOWEVER, if you are looking for a decoder that does, Atlas' is very nice. Their dual-mode decoders can be utilizied on either DCC or DC layouts--the best part is, you don't have to do anything! It will run on either right out of the box (if installed on an Atlas loco). I have one locomotive like this, and it runs like a gem on both DCC or DC. I would highly recommend the dual-mode decoder if you aren't looking for anything fancy.
Hope this helps. :thumb: