|Preach to the choir|
|Crux of the matter|
|Speak of the devil|
|An act of faith|
|—Ambrose Bierce, The Devil's Dictionary (1911)|
Every organized religion in the world has individuals or groups who are there to guide congregations of followers through the trappings of their faith. By and large, their primary roles are similar: performing the rites and ceremonies that a particular religion requires, such as birth rituals, coming-of-age rituals, death rituals, and whatever else the religion tosses in. They are also the keepers of religious tradition and often take on the role of moral leader as well as teacher or "guide."
The term "clergy" describes official religious leadership in a religion. Those within the clergy are referred to as clergymen, priests, and clerics, among other titles depending on religion and ranking.
This page identifies some of the major categories of religious leaders and compares their roles.
In addition to just guiding a religion's followers and performing rites and ceremonies, priests have been given authority to perform sacred acts for the population, acting as a link between the masses and their deity. One of a priest's most important functions is to perform sacrifices, whether human or animal, to a god or goddess. The Judeo-Christian YHWH, (or IHVH, in Latin), for example, required ritual animal sacrifices, until He performed the "Ultimate Sacrifice" and killed His son.
For much of its history, the Jewish religion was administered by priests, a class of Jewish citizens who were given social, economic, and political power far beyond that of just performing the sacred rites of the people. They were second in power only to the kings; when the Romans took over, it was the priestly class that they were most worried about. Abraham is considered the first Jewish priest, though the line formally began with his great-grandson, Levi.
Of course, no women were allowed to be priests 4,000 years ago.
In modern times, Jewish rites are performed by the rabbi, a Jewish scholar and teacher. A rabbi is specifically not a priest, as the priestly class (the Kohanim) still exist, and while they still must abide by certain restrictions, modern Jewish priests serve few special functions. As with clergy in the other Abrahamic religions, men have traditionally held the role of rabbi, and women are still not permitted to be rabbis in Orthodox Judaism, although other forms of Judaism now accept women rabbis (Rabbi Julia Neuberger is a notable example). 
To become a rabbi, one must attend formal schooling. Rabbis oversee community rituals such as the bris, weddings and funerals.
Churches of sacramental traditions (including Roman Catholic, Orthodox, Oriental Orthodox and Anglican Communions, some Lutheran churches, and multiple smaller communities) maintain a priesthood. It usually comprises of three "holy orders" (bishop, priest/presbyter, and deacon) and is closely linked to the concept of "apostolic succession." As a norm, priests lead congregations, while bishops administer groups of parishes (diocese/eparchy) and ordain the priests and deacons. Priests are empowered by a bishop to perform major sacraments, and also to perform additional rites like the blessing of holy water. Deacons help the bishops administer church tasks. Bishops of important dioceses (archdioceses) are called "archbishops," and usually have supervisory duties over bishops within the archdiocese. Within the Roman Catholic Church, some (typically archbishops) are also cardinals, whose powers include electing a Pope when the old one dies or resigns.
Somewhat controversially, Catholic priests make promises of celibacy. Orthodox, Episcopalian, and Anglican Churches can ordain married men as priests (although an Orthodox Bishop must be celibate). Requirements to become a priest vary widely between communities, usually including some form of specialized training. For major denominations in North America, training is provided in seminaries and is usually offered as a graduate degree (Master of Divinity). A candidate to priesthood is ordained by the bishop through the laying-of-hands ceremony (in most cases, candidate is ordained as a deacon first, often only a few days prior). In Catholic theology, this leaves an "indelible mark" on a person, making him a priest for life. The Church can and does punish priests for disciplinary or doctrinal misdeeds by forbidding them to act in priestly capacity. In Orthodox churches, a person can be "defrocked," removing a previous ordination.
Catholic and Orthodox priests must be male. Women cannot hold positions of religious authority in these sects, except leading a community of nuns as an abbess. Anglican and liberal Lutheran churches ordain women priests and at least some branches have women bishops.
The Church of Jesus Christ of Latter Day
Sins Saints has an elaborate concept of priesthood with multiple degrees, based on their own idea of apostolic succession with authority derived from Joseph Smith, who was allegedly ordained by angels. In essence, every male teenager in good standing in the LDS is ordained in the junior ("Aaronic") priesthood order.
Ministers, reverends and preachers
One of the main changes brought about by the Protestant Reformation was the removal of a specialized set of men who were the only people able to intercede between the faithful and God, and the institution of the formal position that each person's relationship with God was direct and personal. This is related to the concept of the priesthood of all believers. But there was still a need for someone to perform rituals, teach the faithful about their faith, and generally be a moral pain in the butt for the "sinners."
There is no real distinction between the titles minister, reverend, preacher, pastor etc. since many different terms describe effectively the same roles in their individual churches. Unlike their Catholic brethren, they are allowed (or in some cases even required) to be or have been married, they do not have the ability to perform supernatural acts, and they do not (despite what individuals might claim) hold ultimate authority over anyone, theologically. They are guides, and as the titles suggest, should be held in high honor, but are not themselves a special set or sect of people. Anyone can call themselves a minister without any formal training or organized rite and legitimacy. However, the larger denominations of Protestant Christianity require formal advanced theological training, usually at a university. Also, as there is no formal "Protestant Church", each individual church or denomination may or may not vest legitimacy in other Churches' ministers.
An imam is a Muslim religious leader. Like preachers and ministers, he guides people in their religious path, teaches them about the Qu'ran, and sometimes performs rituals. In theory, Sunni imams hold less religious power than Shi'a imams, functioning as a leader in prayer and as a community leader, rather than a moral guide. This, however, depends greatly on the local community.
Anyone can become an imam. One does not have to attend special schools or be appointed by the larger religious community. In Sunni Islam, imams are chosen for their leadership and merits and through community consensus while Shi'a imams are believed to be appointed by God
Imams are almost universally male. Only recently have any women stepped into the role, always in so-called western style mosques, and always bringing up serious controversy where it occurs.
Monks and nuns
Monks and nuns are people who choose or are pressured to devote their lives to religious worship, and set themselves aside (at least for a given time) from the rest of society for such solitary practice. While they may reside with others, the core tenet of being a monk or nun is this personal, solitary focus on devotion.
In Christianity, monks and nuns are individuals who wish to devote their life to the worship of God. While they may serve as teachers, unless they are ordained they are not considered to have any of the intercessory powers of priests. Their religious activity is personal and private.
Monks and nuns in Buddhism not only maintain the solitary devotion of monks in general, but they act (for a short term or for a lifetime) as a community's connection to the divine, performing various personal rituals and celebrations for births, marriages, funerals, and the sick. However, they do not "lead" the community in these prayers and rituals as a priest or minister might do; they perform them in solitary, as a way of honoring the person being celebrated. The family may sit around the monk to share and watch the experience, but it is generally not a group activity.
Both women and men perform these rites and rituals — the first order of Bhikkhunis (female monks) being established by the Buddha himself.
Women in Clergy
As most of the world's major religions, and all of the Abrahamic religions, come from a tradition of patriarchy (and even misogyny in most cases), the role of women in the clergy is often controversial. There is a suggestion that in the early church before Constantine women were sometimes leaders and even priests.  Despite this for Roman Catholics and conservative evangelicals it is unacceptable for women to be in any official religious role (priests or ministers). Women may lead prayers but again not from an official religious position, just as a speaker offering testimony. General society outside these fundamentalist denominations and sects is becoming increasingly intolerant of gender discrimination, therefore conservative, male-dominated Evangelical groups are likely to face increasing difficulty recruiting new members.
Since the late twentieth century all major protestant denominations allow women to be ministers. There is a growing trend of seeing women in the ministry, supposedly offering what is often called a more "introspective, personal, healing touch" compared to an older, more patriarchal "do what I say" ministry popular before the 1970s. The same report suggest that male ministers have been strongly affected by their female counterparts, far more willing to open up an emotional side of ministry.
There is a major dispute in many denominations over women becoming bishops which puts them in authority over lower-ranking male clergy.
- Sacrifices used to be vital to ensure, among other things, the sun coming up and the crops growing, but gradually fell out of favor and became rare as the world began to realize how unnecessarily barbaric the practice was.
- The task facing the new chief rabbi
- Catholic Answers: Bishop, Priest, and Deacon
- Catholic Answers: Celibacy and the Priesthood
- Kursk Root Hermitage: Orthodox priests
- Boston College School of Theology and Ministry: Master of Divinity — an example of how to receive one's M.Div. and become a priest
- Catholic Answers: Can priests ordain priests?
- Orthodox Church in America: The Statute of the Orthodox Church in America Article XI
- ReligiousTolerance.org: Women as clergy
- Rev. Elizabeth Eaton, First Female Lutheran Bishop (ELCA) Elected Another woman bishop appointed, as Horsham changes his view
- Encyclopedia of Mormonism: Ordination to the Priesthood
- Reverend, actually, is not strictly speaking a title, but an adjectival epithet. A religious leader may be introduced as "the Reverend Random J. Person," but they would then be addressed as "Dr. Person" (if they hold a doctorate of divinity), or as "Monsignor Person" (if that happens to be their title), but not as "Reverend Person."
- Congressional Research Service: Islam: Sunnis and Shiites Christopher M. Blanchard, 28 January 2009
- The Siasat Daily: Women imams to lead prayers in mosques 28 May 2010
- Book of the Discipline, Pali Text Society, volume V, Chapter X
- With New Pope, Catholic Women Hope To Regain Church Leadership Roles
- International Association of Women Ministers
- ReligiousTolerance.org: Status of women