Exactly what are the "heavenly hosts," as in Luke 2:13? Are these choirs of angels?

That's what I used to think, probably because of the Christmas hymn "Oh, Come, All Ye Faithful", which contains the line "Sing, choirs of angels..."

Yet the word "hosts" is military. These angels are scary—awesome, powerful, dangerous. At the time of Christ's birth their presence accentuated the stunning chasm between the majesty of heaven and the lowliness of the shepherds and infant Jesus. Yahweh is the Lord of Hosts—in command of celestial armies.

In the scene in Luke 2:13, these powerful warriors are praising God. So yes, to answer your question, they are a sort of choir—but no ordinary one!