Pleading refers to the filing of a complaint on the part of a plaintiff or a prosecutor and the response from the defendant. Regardless of the nature of the legal action, it is the first step that must be taken before the commencement of any such action.
In the complaint of a civil action, the plaintiff states his/her case. In legal terms, this is called stating the cause of action, or explaining to the defendant and the court the reasons for taking legal action. A criminal complaint (more accurately known as the charge) is similar, but differs in that it must explain by statute exactly how the actions of the defendant (or accused) were in violation of a specific law.
Until about 160 years ago, pleading in civil cases was done under common law which recognized only certain causes of action, i.e., trespass, debt, covenant (promises made in regards to real estate transactions), ejectment (removal of someone who is occupying property), etc. The problem with this system is that a plaintiff was required to fit the evidence to one of these forms of action. If this could not be done, the case was dismissed.
Common law pleading was abolished in favor of code pleading starting in the 1850's. It had the effect of creating consistency for all types of actions, but because it required a great deal of time-consuming research, plaintiffs would find that statutes of limitations had expired before they could bring their complaints before the court.
Today, most pleading in the U.S. consists of notice pleading. Federal Rules of Procedure now require that a plaintiff need only state the cause of action succinctly in writing; any details involving specific codes or statutes are dealt with in the course of the trial as they become relevant.