Yoma, Chapter 7, Mishnah 5. The Mishnah lays down a rule concerning the priestly garments: which garments are unique to the Kohen Gadol and which belong to the ordinary kohen. The Kohen Gadol serves in eight garments, while the ordinary kohen serves in four alone.
"Kohen gadol meshamesh bishmonah kelim, v'hahedyot b'arba'ah" - the Kohen Gadol serves in eight garments, and the ordinary kohen in four:
The four garments in which an ordinary kohen serves are: "b'chesones u'michnasayim u'mitznefes v'avnet":
"Kesones" - a tunic.
"Michnasayim" - trousers.
"Mitznefes" - a headdress.
"Avnet" - a belt.
The Kohen Gadol wears these four basic garments, and adds four more to them - "choshen v'efod u'me'il v'tzitz":
"Choshen" - the breastplate, which held the stones called the Urim v'Tumim. Inside them was placed a piece of parchment bearing the Name of Hashem, and its power was that it caused the stones to light up, and through this the Kohen Gadol was able to answer those who came to ask what to do.
"Efod" - a kind of apron.
"Me'il" - a sort of outer tunic that he wore.
"Tzitz" - a plate of gold that rested on his forehead, inscribed with the words "Kodesh LaHashem" (holy to Hashem).
Inquiring of the Urim v'Tumim:
When the Kohen Gadol is wearing all eight garments, one may inquire of him through the Urim v'Tumim. The Urim v'Tumim are, if one may put it this way, the 'batteries' of the choshen - they are what enable the stones to light up; and it is a condition that the kohen be wearing all of the garments, so that he can be asked and can interpret the illuminated stones.
Not every person has permission to ask, as the Mishnah says: "v'ein nish'alin ela l'melech, u'l'veis din, u'l'mi shehatzibur tzarich bo" - one whom the entire community, the entire nation, needs, whether for his help or for his leadership. Only these may inquire of the Urim v'Tumim, and no one else may do so.
In summary: in this Mishnah we learned the rule regarding the priestly garments - the ordinary kohen serves in four garments (kesones, michnasayim, mitznefes and avnet), and the Kohen Gadol adds four more to them (choshen, efod, me'il and tzitz); and also that inquiring of the Urim v'Tumim depends on his wearing all of the garments, and is done only for a king, for a beis din, and for one whom the community needs.