The "Homily" is treatise given during the Catholic Mass in which the priest discusses the readings of that day (old testement, epistle, and gospel readings).
Father may discuss how the daily readings from the Bible relate to issues of the day and other moral and religious points he wishes to make.
Homilies are the kind of preaching that was used by the Apostles and Fathers in addressing the faithful. Homlies are expository of the Word (Sermo in latin) of God and therefore are not considered a sermon, the Word of God itself.