George E. Ganss, the translator (who presumably writes the introduction), offers this explanation:
In the Western Church, the sermon (sermo) and homily (homilia) were often interchangable. They seem to be so used in the titles of the printed collections which have come down to us: "Sermons of St. Peter Chrysologus" and "Homilies of St. Valerian."
However, since the time of Origen (186-254 or 255), a distinction has been current between loÃgoß (sermo, discourse, sermon) and oJmiÃlia (homilia, homily). The term sermon is generally used to designate an artisitic production, and homily to denote an informal discourse. A sermon generally develops some definite theme; a homily explains or comments on a passage of Scripture. The sermon usually deals with a doctrinal or moral subject, and is more likely to contain a structural form of introduction, body and conclusion such as textbooks of rhetoric advocate. The homily is more likely to lack structural form, and move or even digress wherever the text leads the preacher. Generally, its purpose is to explain the literal meaning of the Scriptural passage, point out moral or ascetical applications, and perhaps develop accommodated or allegorical meanings.
If we should follow this terminology, we could well reverse the titles which appear on our current Latin editions. Most of St. Peter's discourses are homilies giving a running commentary on a passage (lectio) of Scripture. St. Valerian's discourses usually take their departure from one verse of such a passage, but their nature is far more that of sermons treating a definite theme.