(From greatest to least importance)
Decree - Reserved for dogmatic definitions, beatifications, and canonizations
Apostolic Constitution - Issued in the name of the pope, these may contain defined dogma, but also address Canon Law and Ecclesial Form.
Encyclical - Doctrinal instruction issued to all the bishops, but directed to the universal Church in total.
Apostolic Letter - Typically issued in response to administrative concerns.
Declaration - Focused on Church Law, can be used to clarify, enshrine, or modify existing laws.
Motu Proprio - A decree issued by the sole discretion of the Holy Father, and not in consult with the bishops. The equivalent of an "Executive Order" in political parlance.
Apostolic Exhortation - Doctrine is not defined, but the people are spurred to action regarding the subject matter.