Catechism of the Catholic Church §2101 — excerpt
In many circumstances, the Christian is called to make promises to God. Baptism and Confirmation, Matrimony and Holy Orders always entail promises. Out of personal devotion, the Christian may also promise to God this action, that prayer, this alms-giving, that pilgrimage, and so forth.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.