A Cease and Desist Letter is a formal warning to an individual or corporation to stop ("cease") and not continue ("desist") any offensive or illegal activity. The letter explains why such activity infringes on the sender’s rights and gives the recipient a period of time to voluntarily comply with the demand. The letter also states that if the recipient fails to comply within the given period, the sender is prepared to pursue appropriate legal remedies (i.e. file a complaint in court).
There are no special legal requirements to send a Cease and Desist Letter but it’s best to use a mode of service where you can trace whether it was received or not (i.e. via courier). Further, it’s advisable for you to save copies of the letter for yourself in case legal action is pursued later on.