Creed, Encyclical, Decretal, Canon, Bull, etc - What's the difference? In Catholicism, what is the difference between a Creed, an Encyclical, a Decretal, a Canon and a Papal Bull? Are there any other decrees or formal or informal outlines of official church doctrine used by the Catholic church and  are any excluded from or exclusively used for inerrant revelation from God by the Pope?
CreedThis word comes from the Latin verb credo, which means "I believe." A creed is a collection of articles of the faith* which are necessary to believe for salvation; "creed" can also refer to an individual article of faith. A Sovereign Pontiff can draw up a symbol of faith, and he must do so if the faith is endangered by errors; e.g., the Arian heresy prompted Pope St. Sylvester I, under the guidance of the Council of Nicea, to draw up the Nicene Creed; or Pope Damasus to approve the Nicene-Constantinopolitan Creed of the First Council of Constantinople. Other creeds include the Apostle's Creed and the Athanasian Creed.*The collection of articles of the faith is also called a "symbol," from "συμβάλλειν," which means "to throw together," i.e., a collection.
EncyclicalThese "circular letters" are circulated globally, to all the faithful, and they are meant to express the mind of the Pope. They are not per se infallible, but they are infallible wherever they reiterate previous infallible teaching. Pope Leo XIII was the first pope to write encyclical letters.Pope Pius XII, in his encyclical Humani Generis (1950), wrote, concerning the assent the faithful must give to encyclicals:20. Nor must it be thought that what is expounded in Encyclical Letters does not of itself demand consent, since in writing such Letters the Popes do not exercise the supreme power of their Teaching Authority. For these matters are taught with the ordinary teaching authority, of which it is true to say: "He who heareth you, heareth me";[Luke, X, 16] and generally what is expounded and inculcated in Encyclical Letters already for other reasons appertains to Catholic doctrine. But if the Supreme Pontiffs in their official documents purposely pass judgment on a matter up to that time under dispute, it is obvious that that matter, according to the mind and will of the Pontiffs, cannot be any longer considered a question open to discussion among theologians.
DecretalThis is a somewhat more archaic term that refers to a papal decree, e.g., an official response to a question of discipline.
CanonThis word has several meanings (see them all here), depending on the context, but etymologically it comes from the Latin and Greek meaning "rule." In the present context of "official church doctrine," "canon" means "a short definition of some dogmatic truth, with attached anathema, made as a rule by general councils" (source), which are approved by a Pope. For example, this is one of the canons of the First Vatican Council (1870):1. If any one shall say that human reason is so independent that faith can not be enjoined upon it by God: let him be anathema.and one from the 24th Session of the Council of Trent (1563):Canon II.—If any one saith, that it is lawful for Christians to have several wives at the same time, and that this is not prohibited by any divine law: let him be anathema.
Papal BullThis word comes from the Latin bulla, which refers to the stamped seal on the letter that shows it's authentically from the Pope. Thus, a papal bull is simply an authoritative papal document. Here's the leaden bulla of Pope Urban V:and the bulla of Pope Urban VIII attached to his letter:


Popes can use the medium of bulls, encyclicals, apostolic constitutions, or briefs to speak ex cathedra (i.e., infallibly).**cf. Sixtus Cartechini, S.J.'s 1951 work De Valore Notarum Theologicarum (On the Value of the Theological Notes), which confessors have used when dealing with erudite penitents. It's also available in Italian translation. What I mention here is discussed on PDF p. 17 of the section "Chiarezza e volontà di definire" (Italian edition) or p. 19 of the Latin edition.

See also my answer regarding when a Pope is infallible.