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 terms that people use to describe a church's leadership.
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 version. Note that these are guidelines, and additional tradition-specific versions should be consulted!
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 for the right hand, the one you would use to give something to someone else (as opposed to your sinister left hand, which was typically used for less pleasant duties). 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 effective in furthering the kingdom
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.
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: