Typically when a priest is newly ordained, he will be assigned as a parochial vicar at a parish.
What is a parochial vicar?
A parochial vicar is an assistant priest who works closely with the pastor of a parish, helping him in every way.
The Code of Canon Law explains this type of assignment.
Whenever it is necessary or opportune in order to carry out the pastoral care of a parish fittingly, one or more parochial vicars can be associated with the pastor. As co-workers with the pastor and sharers in his solicitude, they are to offer service in the pastoral ministry by common counsel and effort with the pastor and under his authority.Can. 545 §1
Often this is a way for a newly ordained priest to learn from an experienced priest and to help a pastor who has a large parish.
Normally there is only one parochial vicar, but in larger cities there can be several vicars and sometimes they are assigned to multiple parishes.
This type of assignment is nearly always temporary, lasting only a few years, until that priest is ready to become a pastor of his own parish.