The difference between a pastor, minister and reverend is that reverend is a title given, but not a position, while a minister is a minister of religion and a pastor means the same thing as a minister. Other terms to describe Christian leaders include elder and bishop.
An elder was the title used in the Jewish community and is rarely used in the Christian community today, but can be. It refers to a person who is a spiritual advisor due to his or her age. A bishop is an overseer who supervises several churches. The pastor or minister is the spiritual overseer or spiritual leader of his church. A pastor or minister will only serve one church at a time and will lead that church in their weekly services as well as extracurricular church programs. Sometimes, the pastor or minister will be addressed as a reverend. The exact title used for the position varies from church to church and denomination to denomination.
The title of pastor actually came to be because the word itself means "shepherd." This is a metaphor that is seen in the Bible when it is said that God leads a flock of sheep, which are his Christian followers. The term "pastor" is used similarly here as it means that the pastor is shepherding his flock of churchgoers towards Christ and eternal salvation.