The ‘‘‘Pallium’’’ or ‘‘‘Pall’’’ is an ecclesiastical vestment used in the Church of Rome and originally worn only by the Bishop of Rome. However, it has been bestowed by him for centuries on metropolitans and primates in the Western Church as a symbol of the jurisdiction delegated to them by the Holy See.
In its present form the pallium is a narrow band of clothe, "three fingers broad," woven of white lamb's wool from sheep raised in Valencia, Spain, with a loop in the centre resting on the shoulders over the chasuble, and two dependent lappets, before and behind. Thus, when seen from front or back the vestment resembles the letter Y. It is decorated with six black crosses, one on each tail and four on the loop, is doubled on the left shoulder, and is garnished, back and front, with three jeweled gold pins.
In origin the pallium and the omophorion, used in the Orthodox Church, are the same vestment. The omophorion is a wide band of cloth, much larger than the modern pallium, worn by all Eastern Orthodox bishops and Catholic bishops of the Byzantine Rite.
Only the Pope and metropolitan archbishops wear the pallium. A metropolitan has to receive the pallium before exercising his office in his ecclesiastical province, even if he was previously metropolitan elsewhere. No other bishops, even non-metropolitan archbishops or retired metropolitans, are allowed to wear the pallium unless they have special permission.
-  Wikipedia: Pallium