C&D is a Cease and Desist. Basically, it's when the lawyer of someone who has intellectual property contacts you and tells you 'stop doing this, or we're taking you to court', but in more legal language. Usually most fans will stop whatever it was then*, because even the risk of civil court is kind of scary.
* Or their webhosts will tell them to stop or get kicked off the server. Webhosts don't want to deal with legal issues much either.