What is the difference between a pastor, a priest, and a minister?

The English words you cite come with an awful lot of denominational baggage.

The word minister was originally used to denote someone who served others (ministered to them).

Matthew 20:28 Even as the Son of man came not to be ministered unto, but to minister, and to give his life a ransom for many.

minister = diakoneō = to be an attendant, to wait upon

The word priest was used to describe someone in the old testament who offered sacrifices in the temple. Hebrews describes Jesus who fulfills all the requirements of the priesthood. Peter refers to believers as a royal priesthood.

The word pastor is used only once in the new testament as a noun in Ephesians and it is grammatically joined with the word teach such that the idea is given that shepherding is accomplished by teaching.

An etymology-based perspective

Here are some meanings for these roles based on the original meanings as used in the New Testament. Many denominations attempt to use these terms in the same way, such as the Brethren, Presbyterians and some Baptist churches.

Pastor: a shepherd. Someone who looks after the 'flock' of God: his people. Pastors are listed in Ephesians 4:11 as one of the gifts that God has given the church. There are varying opinions about gifts: some think this is a leadership role, and others that it describes regular church members - in both cases they would be the people who are naturally good at caring for the spiritual health of the other church members.

Priest: someone who mediates between a god and people. Jesus is called our Great High Priest, while many Protestants support a doctrine called the Priesthood of all Believers, which is based on 1 Peter 2. Those who support that doctrine believe that all Christians are priests because we are all meant to go between God and the people around us, and therefore the leaders of a church should not be called priests because they are not special in that regard.

Minister: a servant. (We may not think of it that way, but in politics a Prime Minister is the head of the Public Servants, so the meaning is still hidden there.) There is a sense in which all Christians are meant to serve each other, and so we are all ministers, and we all do ministry. There is another sense for people who are particularly gifted at serving others - this is the sense found in Romans 12:7. And there is another sense: the Greek word for 'minister' is 'diakonia', from which we get the word Deacon. Most denominations believe that a Deacon is a person who has been specifically appointed to a position. Passages like 1 Timothy 3:8-13 might be indicating an appointed position, or they might not - I'm not particularly sure either way.

And though you didn't ask for this one, I'll include it too:

Elders, Presbyters and Bishops: Presbyter is a transliteration of the Greek word meaning Elder. The idea is like a village elder, the natural leaders of a small community because of their age, wisdom and experience. In most Protestant denominations except for the Anglicans Bishops are thought to be equivalent to Elders - the word Bishop means an Overseer (which is how some Bibles translate it.) The idea is that they are there to oversee the church, to "hold firmly to the trustworthy message as it has been taught, so that he can encourage others by sound doctrine and refute those who oppose it." (Titus 1:9)

None of the terms I have described necessarily imply an employed position in a church. Most churches will use one of these terms and use it to mean that the person is an employed staff member of the church, which can give the wrong idea that their title is used because they are employed rather than because they are a servant or an overseer etc.

Some of the primary differences are due to the believed place of the clergy.

In some churches being clergyman is considered a "vocation". If someone is called to be ordained they are ordained for life (barring exceptional circumstances). Even if they stop doing the job of a priest, they remain one. The denominations that practice this typically have a three-level clergy: bishops, priests and deacons. These churches include Roman Catholics, Orthodox, Anglican and Lutherans. In these denominations:

• Priests are the primary ministers, normally responsible for overseeing the congregation
• Bishops are overseers of other clergy
• Deacons are either more junior or more specialized clergy. In Catholic and Anglican traditions a deacon cannot celebrate Mass.

Reverend is a title applied to one of these e.g. "Reverend John Smith". To call someone "a reverend" is technically incorrect.

Other protestant churches reject the idea of ordained clergy. A church leader is appointed to a specific job, and can leave it. When a pastor quits his job he is no longer a pastor. These churches use different terms for their leaders:

• Pastor From the word for shephered, one who oversees and provides care for the congregation. Many churches reserve the title for their more senior leaders within a congregation.
• Minister from the word for servant. One who provides for the needs of the congregation, and typically used for church leaders. Often interchangeable with pastor.
• Preacher one who preaches. May or may not also be a church leader.
• Apostle originally refers to one of the Twelve chosen by Jesus, and then to other people. It literally means "one who is sent", and is applied to someone who carries the Christian message to others. Wikipedia has a good article. Some churches still use the term. The Latter Day Saints use the term to refer to their highest leaders.
• Elder AffableGeek describes this much better than I could. It depends on the tradition, but is frequently used to describe a member of an advisory or leadership committee within the church, separate from the pastors.
• Deacon is sometimes also used by these churches for more junior leadership roles or more junior Elders.

Other terms:

• Vicar in the Catholic church is a priest who is the bishop's helper. In the Anglican church it is a priest who is in charge of a parish.
• Chaplain normally means a priest or minister who serves a specific community or institution other than a church - e.g a prison, military unit or football team.

An attempt at a generic Protestant version. Note that these are guidelines, and additional tradition-specific versions should be consulted!

Clergy:

• Priest One who conducts the rituals that mediate between God and man.

• Pastor From a word meaning 'shepherd', one who 'watches over' a church, manages it, and tends to its members. Because of the shepherd connotation, the members of the church are often called the flock as in the flock of sheep under a pastor's care.

• Minister One who tends to the needs of others. From the Latin minister=servant, from Latin minus=less. A pastor would by definition minister to his congregation.

• Chaplain A person who is called to minister to a group of individuals on an as-needed basis, such as for weddings, funerals, and hospital visitations only. In some very small congregations, chaplains are also called to serve in part-time basis only, if the church meets less than weekly, or if the pastor is not expected to perform regular visitation.

• Preacher One who actually preaches a sermon

• Reverend A generic title for any ordained persons, one who is to be 'revered' for his special calling

• Bishop An overseer of the clergy, most typically involved in a church hierarchy. Baptists, Mennonites, and other congregational churches who reject hierarchies also reject bishops. In African American churches, this can also refer to any pastor who has raised up pastors within his own church, and thus may effectively be just a very successful senior pastor.

• Apostle Literally one who is "sent out" (from the Greek) to spread the news. Historically, this refers to those 12 men whom Jesus specifically charged with the task of spreading the news. 11 of these Apostles were originally disciples i.e. students of Jesus

Depends on the tradition:

• Deacon Depending on the tradition, can have a wide variety of meanings, although all indicate that one is serving in a capacity. The deacons were so named in Acts 6, in order to relieve the Apostles of duties that were time consuming but not as important as their ministry through proclaiming the Word. Specifically the line (in 6:2) is "It would not be right for us to neglect the ministry of the word of God in order to wait on tables."

  As an example, a Deacon in a Baptist Church is equivalent to an elder in a Bible Church or a vestry member in the Episcopal church - all of which are non Clergy who in effect "run" the local church which the priest / minister / pastor is paid to lead. Think of it as the board to the Pastor's CEO. In contrast, a Deacon in the Episcopal church is a guy who has just been ordained, and in most instances, will become a priest in about 6 months time, but has not yet so been named. As a deacon, they can read the Gospel, prepare the Eucharistic table (but not actually lead the Eucharist), and pronounce the blessing at the end of the service. All of this to say - an Episcopal deacon is a junior priest whereas a Baptist deacon would be a very senior member. As you can see, there is a lot of variation.

• Elder Also highly dependent on the tradition.

  In "Bible Churches", the Board of Elders functions much like a vestry - a board of committed, respected members of the congregation would serve in an advisory or leadership capacity to set the direction of the church.

  In the Mormon tradition, since most men are supposed to preach, the elders have more of an organizing role for the congregation.

Note: In the NT, deacons, elders, and later bishops are the only mentioned leadership positions.