Short answers, pulled from the story.
The word bull derives from the Latin verb bullire, meaning to boil, which refers to the molten state of the metal used to create the seal that authenticated these documents. This etymology reflects the physical process of melting and pressing the metal to form the seal.
No original papal bull survives in its entirety from before the year 819. The oldest surviving lead bullae date back to the 6th century, but the written documents themselves are rare before the 11th century.
The Florentine engraver Benvenuto Cellini was paid 50 scudi to recreate the metal matrix used to impress the lead bullae of Pope Paul III in 1535. Cellini added fleurs-de-lis to the reverse of the seal, a heraldic device of the Farnese family from which Pope Paul III descended.
The phrase papal bull was not used until around the end of the 13th century and then only internally for unofficial administrative purposes. The term was not official until the 15th century, when one of the offices of the Apostolic Chancery was named the register of bulls.
Since the late 18th century, the lead bulla has been replaced with a red ink stamp of Saints Peter and Paul with the reigning pope's name encircling the picture. Very formal letters still receive the leaden seal, such as the bull of Pope John XXIII convoking the Second Vatican Council.