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.
There are three forms of this sacrament: diaconate (deacon), presbyterate (priest) and episcopate (bishop). One sacrament, celebrated three times with successively higher sacramental effects.
Every man in Holy Orders is either a deacon, priest or bishop. An archbishop is a bishop in charge of a large or important diocese called an archdiocese. A cardinal is a special member of the papal household. Nearly all cardinals are bishops. The pope is the Bishop of Rome.