A cease and desist letter is a written warning to a party believed to be doing something unlawful to instruct that person to stop doing the unlawful action. Cease and desist letters are typically written by attorneys. If the party who receives the letter does not stop doing the specified action, then litigation can follow.
People usually request that attorneys send cease and desist letters when they believe that their rights are being infringed upon in some way. Examples include copyright, patent, and trademark disputes. For example, Company A may believe that Company B is using some of their patented technology. Company A may then hire an attorney to send a cease and desist letter to Company B warning it to stop. If Company B refuses to stop, then a lawsuit may occur.