Replying to References on Resumes

this shit has been bugging me for a long time and never really gotten a straight answer.  maybe I should have listened in careers class........

anyways when you put someone as a reference, should you tell that person you did so?  ive always just put their names and shit on my reference and I know its against the law for an employer to give a bad reference, but like maybe they dont want to be called or something?  i dunno ns hit me up with answers.