No, this is not the start of a joke about a bunch of people walking into a bar. Rather, I thought it would be helpful to clarify various titles that people give to leadership roles in churches.
Priest The biblical order of presbyters. One who conducts sacrifices
Pastor The priest who is in charge or a parish, he may have associate pastors - recently ordained priests start as associate pastors.
Minister Ordinary ministers are the bishops, priests and deacons who administer the sacraments to the faithful. Extraordinary ministers are laypersons appointed by the priest to help in the administration of the sacraments.
Chaplain An ordained person whose vocation is specific to their task (i.e. hospital chaplain, military chaplain etc..) not to a parish.
Preacher Not a specifically Catholic term.
Reverend A title given to any priest deacon or bishop (to varying degrees)
Deacon An ordained person in the order below priests. Deacons cannot celebrate Mass or administer the sacrament of reconciliation (strictly speaking, they can hear your confession - but they can't do anything about it). Deacons come in two flavors. The permanent deaconate (people ordained specifically as helpers to the priests) and transitional deconate (seminarians ordained a few years before their eventual ordination to the priesthood)
Elder Not a specifically Catholic term.
Bishop The successors of the apostles who oversee their diocese or bishopric (in the Eastern Catholic churches). Together with the Pope, the Bishop of Rome, they are the top of the Catholic hierarchy. They are the ones who can confirm with the Holy Spirit (or appoint priests to do so in their stead) and only they can ordain priests.
An attempt at a generic Protestant version. Note that these are guidelines, and additional tradition-specific versions should be consulted!
Depends on the tradition:
Note: In the NT, deacons, elders, and later bishops are the only mentioned leadership positions.
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:
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:
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.