Sacraments are holy, visible signs instituted by Christ of an invisible reality, in which we can experience the healing, forgiving, nourishing, strengthening presence of God that enables us to love in turn. In traditional Catholic theology, the effectiveness of a sacrament depends both on the action of the Spirit and on the response of the person and community celebrating it - as always, a covenant. Sacraments bring the response-ability to live as graced people.
-from The Essential Catholic Handbook, Liguori and What Makes us Catholic by Thomas H. Groome