Being a priest does not mean just assuming an office or a ministry. Through Holy Orders a priest receives as a gift a definite power and a mission for his brothers and sisters in faith.
A Catholic priest who administers the sacraments acts not on the basis of his own power or moral perfection (which unfortunately he often lacks), but rather "in persona Christi". Through his ordination, the transforming, healing, saving power of Christ is grafted onto him. Because a priest has nothing of his own, he is above all a servant. The distinguishing characteristic of every authentic priest, therefore, is humble astonishment at his own vocation.
The episcopal ministry is the real pastoral ministry in the Church, for it goes back to the original witnesses to Jesus, the apostles, and continues the pastoral ministry of the apostles that was instituted by Christ. The Pope, too, is a bishop, but the first among them and the head of the college.
(Taken from YouCat)