Divine Mercy & Papal Infallibility

Hopefully you'll find the above video on papal infallibility helpful. 

Here is a run through of the different levels of papal pronouncements. It comes from the Mount Saint Mary's Seminary in Cincinnati's website. I left the order that they give in place, although my understanding is that papal bulls and moto propios only have as much importance or weight as the directives that are being given. The usual order of authority is, from highest to lowest, is 1. Apostolic Constitutions, 2. Encyclicals, and then 3. Apostolic Letters and 4. Apostolic Exhortations. As I've learned from doing my research there doesn't seem to be an exact science to this.

• Papal Bull

A bull is a formal papal document authenticated with a bulla or seal. Originally, metal was used for the seal, but today metal is used only for the most solemn bulls. Bulls typically begin with the issuing pope's name and the phrase, "episcopus servus servorum Dei", and conclude with the date. The content may cover any topic. Many important papal documents are issued as bulls. Bulls are generally cited using the first words of their text.

• Apostolic Constitution

Apostolic constitutions are the most solemn form of legal document issued by the pope in his own name. Frequently issued as bulls, they deal primarily with serious doctrinal matters. Since 1911 they have also been used for the establishment of dioceses and provinces. Many important documents have been promulgated as constitutions, including The Code of Canon Law.

• Motu Proprio

Motu proprios are legislative, apostolic letters written and signed by the pope on his own initiative. Originally used to settle the affairs of the Curia and administer the Papal States, they now handle legislative matters which are significant but do not merit a constitution. Motu proprios are generally brief, and handle specific issues relevant to the Church in a specific time in history. In recent years they have been one of the principal sources of new laws outside of the Code.

• Encyclical Letter

Encyclicals are papal letters of a pastoral nature, used in their current form since 1740. These letters offer counsel and shed light on existing doctrine as part of the Holy Father's ordinary teaching authority. They do not belong formally to the deposit of revelation, and their teachings are not definitive unless specifically stated as such. Therefore, certain points of their teachings can often be changed.

• Apostolic Epistle or Letter

When an encyclical is written in response to a particular need, or when it is addressed to a specific group of persons, it is called an apostolic epistle. These epistles, pastoral in nature and primarily discussing social concerns, are not considered legislative documents.

• Apostolic Exhortation

Apostolic exhortations are papal reflections on a particular topic which are addressed to all of the clergy and faithful. The form was first used by Pope Pius XII in 1939. Exhortations generally encourage a particular virtue or mission, and are given on specific occasions. They do not contain dogmatic definitions or policies, and they are not considered legislative documents.

• Decretal Letter

Decretals, first used in the 2nd century, originally contained papal decisions related to questions of discipline and administration. During the Middle Ages they were often issued in the form of bulls. In modern times, they are reserved for solemn matters such as dogmatic definitions and canonizations. It is generally accepted that decretals belong to the extraordinary magisterium of the Holy Father; however, they are not considered legislative documents.

• Allocutiones / Addresses

Prior to the 19th century, the term "allocutiones" was generally reserved for solemn addresses given by the Holy Father to his cardinals. Today, however, a variety of less-formal papal allocutions and addresses are published in the Acta Apostolicae Sedis and other sources. Their content is not necessarily tied to the Church or liturgy, but attempts to address specific issues or circumstances and is aimed at particular groups of people.

• Papal Rescript

A papal rescript generally answers a petition placed before one of the Roman Curiae or the Holy Father himself. It is signed by the cardinal prefect and the secretary of the relevant congregation, and also bears that congregation's seal.

• Apostolic Brief

Apostolic briefs, also called brevia, are a simple form of document dealing with matters of relatively-minor importance. This form emerged during the reign of Pope Martin V (1417-31) as a replacement for the ancient form of litterae.