Hi, Leeneia-
According to Richard McBrien's Encyclopedia of Catholicism, and antiphon is

a short verse, usually from the Bible, sung at the beginning and end of psalms or canticles, or as a refrain between verses...Several antiphons are tied to liturgical processions on special feast days.

But in the days of my youth, "the" antiphon of a Mass was a short verse that was sung or recited at the beginning of Mass, and rarely as a refrain between verses. I believe there was once a psalm with an antiphon at the beginning of Mass, between the scripture readings, and during communion. There is still a psalm between the scripture readings; but the other two psalms were replaced by hymns, athough the vestigial entrance antiphon (introit) and communion antiphon remain as an option, to be sung or recited with or without a psalm or canticle.
"Antiphonal singing" is sung by two choirs, usually alternating verses of a psalm - and may or may not have an antiphon.
-Joe-