In contemporary usage, the term is almost always used as an adjective, for example "three pieces from the set are in concertato style."
A somewhat oversimplified, but useful distinction between concertato and concerto can be made: the concertato style involves contrast between opposing groups of voices and groups of instruments: the concerto style, especially as it developed into the concerto grosso later in the Baroque, involves contrast between large and small groups of similar composition (later called "ripieno" and "concertino").
Different choirs or instrumental groupings occupied positions across the basilica from each other: because of the sound delay from one side to the other in the large and acoustically "live" space, a perfect unison was difficult, and composers found that a fantastically effective music could be composed with the choirs singing across to each other, in stereo as it were; all accompanied by organ or other groups of instruments placed in such a way that they could hear each group equally well.
Often, sacred music in the concertato style in the early 17th century was descended from the motet: the texts that a hundred years earlier would have been set for a cappella voices singing in smooth polyphony, would now be set for voices and instruments in a concertato style.
The concertato style made possible the composition of extremely dramatic music, one of the characteristic innovations of the early Baroque.