As far as Christianity is concerned, the question has been answered. But "priest" is an ancient word whose origins predate Christianity. It refers to a person with authority to minister to sacred things. Today, if an anthropologist refers to a leader of an ancient religion under study, he calls that person a "priest". The notion of "pastor" is also an ancient concept. It refers to a person who tends a flock. The word avails itself naturally to spiritual metaphors, as in Psalm 23 where Man is treated as a sheep in the flock of God. Today, "pastoral care" is used as a synonym for "spiritual care", regardless of the religion. It is true that Protestants refer to their spiritual leaders as "Pastors", and you should use that term when talking to your Protestant friends, but an anthropologist would say that 'Pastor' is merely what Protestants call their priests.

They are both leaders of congregations at different Christian "denominations." For historical and theological reasons, the title used for the leader of a congregation vary. The head of government in some countries is a "president," and in others a "premier" and in others a "prime minister." It's that kind of thing. Traditionally, Roman Catholics are led by "priests." Different Protestant denominations use different terms. "Pastor" is traditional for Lutherans, "minister" for Presbyterians. I think it's more complicated than that, but that's the idea. Literally, a "pastor" is a shepherd, who takes care of and tends a flock of sheep. That is supposed to convey the nature of the relationship when "pastor" is used. Pastors will sometimes even refer to their congregations as "my flock." A Jewish congregation is led by a "rabbi," which means "teacher."

Today, the term "priest" is used in the Catholic Church, Eastern Orthodoxy, Anglicanism, Oriental Orthodoxy, the Church of the East, and some branches of Lutheranism to refer to those who have been ordained to a ministerial position through receiving the sacrament of Holy Orders, although "presbyter" is also used. 不过，有部分华人地区错误把priest译成“牧师”（pastor），容易令人误解圣公会祭司的角色。

Depends on denomination. Priest is associated with Roman Catholicism. Pastors with Protestantism

The details get infinitely complicated, but I think I want to add a third word to the list. In the United States, the most common, most generic word for the leader of a Protestant congregation is "minister." Fortunately, in our government we don't have use the word "minister" for government officials, so there's no ambiguity.