Service of process is one of the first steps in the life of any lawsuit. It can occur in several different fashions, but the most practical, and the most efficient is personal service. Simply put, personal service means that someone (usually a process server) identifies you in person, and hands you the papers that create a formal complaint / lawsuit. It does not matter if you formally accept the papers or if you run away. As long as the process server sees you in person and attempts to hand you the documents, you have been served.
Personal service by a process server is the most efficient manner of service because a process server signs a formal declaration stating the service has occurred and the court almost always accepts this as indisputable proof of service. Once this happens, the Defendant has 30 days to respond to a lawsuit.
If personal service is not possible, which can happen for a variety of reasons, then you may serve the lawsuit by mail. This is not as efficient because it requires the Defendant to sign a piece of paper and mail it back to you. If you are trying to serve someone by mail, then it is likely they are avoiding personal service, so they are unlikely to sign anything and mail it back to you
One of the final steps is to attempt service by publication. This can only occur with approval from the court, and only if other less public methods have failed.