Catechism of the Catholic Church §167 — excerpt
"I believe" (Apostles' Creed) is the faith of the Church professed personally by each believer, principally during Baptism. "We believe" (Niceno-Constantinopolitan Creed) is the faith of the Church confessed by the bishops assembled in council or more generally by the liturgical assembly of believers.
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.