Short answers, pulled from the story.
A papal bull is a formal public decree, letters patent, or charter issued by the pope of the Catholic Church. The name comes from the leaden seal called a bulla, which authenticated the document; the word bulla derives from the Latin bullire, meaning "to boil," referring to the melting of wax or metal to soften it for impression.
The lead seal traditionally depicted the apostles Saint Peter and Saint Paul on the front, identified by the letters SPASPE. Saint Paul was shown with flowing hair and a long pointed beard, while Saint Peter had curly hair and a shorter beard of dome-shaped beads called globetti. The reverse bore the reigning pope's name and the letters PP, for Pastor Pastorum.
The Florentine engraver Benvenuto Cellini was paid 50 scudi in 1535 to recreate the metal matrix used to stamp the lead seals of Pope Paul III. Cellini retained the traditional imagery of the two apostles but carved them with greater artistic detail, and added fleurs-de-lis on the reverse to represent the Farnese family lineage of Pope Paul III.
No papal bull survives in its entirety from before 819. Original papal bulls exist in quantity only from the 11th century onward, when the transition from fragile papyrus to more durable parchment was made. Some original lead bullae, however, still survive from as early as the 6th century.
The phrase papal bull was used informally for internal administrative purposes from around the end of the 13th century. It became official by the 15th century, when one of the offices of the Apostolic Chancery was formally named the register of bulls, or registrum bullarum.
Papal bulls have been used for a wide range of formal communications, including statutory decrees, episcopal appointments, dispensations, excommunications, apostolic constitutions, canonizations, and convocations. One notable example is the bull of Pope John XXIII convoking the Second Vatican Council, which still received a leaden seal.