The exhibition or delivery of a writ, summons and complaint, criminal summons, notice, order, etc., by an authorized person, to a person who is thereby officially notified of some action or proceeding in which he is concerned and is thereby advised or warned of some action or step which he is commanded to take or to forbear.

In Plain English:

Service is the term used for giving notice to someone. For example, when the plaintiff files a complaint in a lawsuit, the plaintiff must also serve the defendant the complaint. The plaintiff can serve the defendant personally, by having a registered person give the papers to the defendant, or by mail. The main goal is to take reasonable steps to be sure that the defendant knows he is being sued. It is an important step, because the Constitution requires that someone who is involved in a lawsuit be put on notice of the lawsuit so that he or she has a chance to defend himself or herself.