Eighty-seven hymns, none long, each addressed to a single divine power and built almost entirely out of epithets: Hekate is of the roads, of the tomb, of the sea, saffron-robed, bull-faced, keeper of the keys. The accumulation is the point. To name a god by every name is to leave no aspect of that god unaddressed, and the hymn ends by asking the power so summoned to come and be kindly.
What marks these out from literary hymnography is the rubric attached to nearly every one, naming the substance to burn with it: storax, myrrh, frankincense, aromatic herbs, and for the powers of the underworld, no incense at all but a libation. Those are stage directions. They indicate that this is the order of service of a real cult association, and the sequence in which the hymns are arranged (beginning at night, moving through the cosmic powers, the Olympians, the nature powers and the chthonic deities), looks like the shape of an actual nocturnal rite rather than an editor's filing system.
The attribution to Orpheus is pseudonymous, in the ordinary ancient manner: a body of ritual poetry is put under the name of the mythical singer whose authority it claims. Composition is placed in the second or third century CE, most probably in Asia Minor, on the basis of language, metre and the make-up of the pantheon.