Personal service is service of process directly to the (or a) party named on the summons, complaint or petition. In most lawsuits in the United States, personal service is required to prove service. Most states allow substituted service in almost all lawsuits unless you are serving a corporation, LLC, LLP, or other business entity; in those cases, personal service must be achieved by serving (in hand) the documents to the "Registered Agent" of a business entity. Some states (Florida) do not require that the documents actually be handed to the individual.
In California and most other states, the documents must be visible to the person being served, i.e., not in a sealed envelope. If the individual refuses to accept service, flees, closes the door, etc., and the individual has been positively identified as the person to be served, documents may be "drop" served, and it is considered a valid service. Personal service of process has been the hallmark for initialing litigation for nearly 100 years, primarily because it guarantees actual notice to a defendant of a legal action against him or her. Personal service of process remains the most reliable and efficacious way to both ensure compliance with constitutionally imposed due process requirements of notice to a defendant and the opportunity to be heard.