The cassock is the oldest surviving item of Christian clerical dress in continuous use, and it survives for an unremarkable reason: it is simply the long tunic that ordinary men in the late Roman world wore, kept in use by the clergy after everyone else moved to shorter garments. Clerical dress is generally conservative in exactly this way, preserving the ordinary clothing of the period at which it was fixed.
The Roman form fastens down the front with a long row of buttons, and a widespread tradition counts thirty three of them for the years of Christ's life, with five more on each cuff for the wounds. The count is devotional rather than prescribed and cassocks are made with other numbers.
The Anglican Sarum form is double breasted, fastening at the shoulder, with a fabric belt. Over the cassock a fascia or cincture, a broad sash, may be worn, and its colour follows rank in the same way the cassock's does. A simar adds a shoulder cape. For formal non liturgical occasions a bishop may wear a ferraiolo, a full silk cloak.
Eastern practice distinguishes two garments where the West has one. The inner rason, called an anteri or podryasnik, is the close fitting robe; the outer rason, the exorason, is a wide sleeved black overgarment worn over it, and it is this that gives Orthodox clergy their characteristic silhouette. Monastics wear the same garments, and a bishop, always a monk, adds a mandyas for processions.
The cassock is not a vestment and confers nothing. It is worn under the alb at the altar and on its own in the street, and in most of Western Europe it disappeared from the street within a generation of the 1960s.