Dear Liz, 

What priests, sisters and consecrated persons have in common is the fact that they commit themselves freely, out of love, to follow Christ as perfectly as possible in celibacy for the Kingdom of God, and they place themselves at God's service through obedience. The religious and consecrated souls also take a vow or promise of poverty. The priest of course is set apart by the Sacrament of Orders to offer the sacrifice of the Mass and take people's sins away through the sacrament of Confession. The Sacrament of Orders transforms him and makes him able to take Christ's place and do both these in his place. 

God bless.