The typical, traditional way to start a divorce is one party personally serving the other party with a divorce summons and petition. That is, one of the parties must be literally handed the documents. These days, it is at least as likely that the receiving party will simply sign an Admission of Service when the documents are emailed to them. For the responding party, that prevents the awkwardness of receiving personal delivery of legal documents at home or, worse yet, at one’s workplace; and for the petitioning party, it avoids the hassle and expense of carrying out the task of personal service.
A joint petition is different. Both parties sign a joint petition, which does away with the need for a summons, and also does away with the need for personal service of the documents. If both parties cooperate in commencing the case (which requires both parties to assert that there is an irretrievable breakdown of the marriage), then neither party needs to serve the other party.
Oftentimes, parties who proceed on a joint petition can also agree on the terms of the divorce, and incorporate the agreed terms into the joint petition. In that case, the entire divorce case is basically encapsulated in one document. This is most likely to happen if the marriage is of relatively short duration, and there are no children of the marriage and no complicated financial issues.