A hazzan leads congregational prayer through chant, and the role sits at the intersection of two different obligations that hazzanut as an art form has to hold together. The office developed as synagogue prayer took shape, after the destruction of the Second Temple, as Judaism's central form of collective worship in the absence of Temple sacrifice, and a dedicated prayer leader trained in the tradition's chant became a natural and enduring feature of that new liturgical center. The first obligation is fidelity to nusach, the traditional melodic modes assigned to each service and occasion, which a hazzan is expected to know and apply correctly so that a congregation recognizes the service being prayed by its melodic character alone. The second is vocal artistry: within the framework nusach provides, a hazzan is free, and in many communities expected, to extend and ornament the traditional material, showing off breath control, range and expressive phrasing in a way that a plain reader of the same words would not.
This is distinct from cantillation, ta'amim, which governs the fixed melodic reading of scriptural text itself rather than the sung leadership of prayer more broadly; a single service can involve both a hazzan chanting the surrounding prayers in nusach and, separately, cantillation of a Torah or haftarah reading within it, performed by the same or a different reader according to community custom.