The Sacrament of Holy Orders is the continuation of Christ's priesthood, which He bestowed upon His Apostles. The three levels of the sacrament are the episcopate, the priesthood, and the diaconate. In the Sacrament of Holy Orders, or Ordination, the priest being ordained vows to lead other Catholics by bringing them the sacraments (especially the Eucharist), by proclaiming the Gospel, and by providing other means to holiness.
Source: Catechism of the Catholic Church