The word Clergy is derived form Greek word kleros which means inheritance. Prime responsibilities of clergy includes taking over religious duties which include birth and death ceremonies, baptism, religious programs, etc.
Unlike other careers, formal education is not mandatory, instead one needs to be trained under a religious leader like in a gurukul. Some religions require clergymen to be learned about various practical aspects of the religion. Some of the universities in US provide a degree in Theology to train diocese priests, later which they're designated as assistant pastor in the church.