Deriving from the word for “sending” or “messenger,” and the word for “community,” the sheliah tzibbur is the person who leads the congregation (the tzibbur) in prayers. Sometimes the sheliah tzibbur is considered to be fulfilling obligations on behalf of the congregation.
The Shulhan Arukh states that the person must be “appropriate,” which has been further defined as being free from sin, about whom there must never have been ugly gossip spoken, humble and desired by the community, having a pleasant voice, and being able to regularly read from the Torah, Prophets, and Writings. It also notes that if one cannot find such a person, the congregation should choose the best of the community in matters of wisdom and good deeds.
The Mishnah Berurah says that the sheliah tzibbur should be the first into the synagogue and the last one out, not foolish or frivolous, and should be able to speak of the needs of the community. Mishnah Taanit 2:2 suggests that it is critical for the success of the community’s prayers for them to choose a qualified sheliah tzibbur, mentioning an old experienced person who had had children but whose house is now empty such that prayer would be the heart’s sole devotion.
Others who lead prayer are the hazzan and the ba’al tefillah (“master of prayer”). The position of the hazzan seems to have grown out of the sheliah tzibbur tasks. The hazzan, or cantor, was expected in the earliest times to have not only a knowledge of the literature of the Bible and liturgy, but also the prayer motifs, and a pleasant voice and artistic delivery. It is only in the last few centuries that being a hazzan became a full-fledged profession, and was further considered to be a clergy position, even by European governments after Jews were accepted as citizens. In fact, the U.S. government recognized cantors as the first Jewish clergy even before recognizing rabbis. Hazzanim of course perform many more functions - e.g., in earlier times they were payyetanim, prayer writers, and they also often added even more tasks, learning for instance to serve as mohels.
The sheliah tzibbur is a lay person. It is an honor to be asked to lead prayer, and humility is supposed to dictate first refusing, then getting ready, and on the third offer accepting.