In "Doctrinal Discussion" William Sillings writes:
While gospel songs are forms of praise, traditionally hymns have carried the greater weight of praise. Defining a hymn is an elusive task, but most authorities, says Ralph Martin, appeal to Augustine. Augustine claimed there were three distinctive characteristics of a hymn. 1. A hymn is praise. 2. It is designed to be sung. 3. It is directed to God. Furthermore, a hymn differs largely from a gospel song in that the gospel song tends to be more subjective. That is, it tends to concentrate on “my experience” of certain great spiritual truths. In contrast, the most remarkable characteristic of a hymn is its objectiveness. Hymns express in virtually universally applicable terms the praise of the Church to God, whereas a gospel song may describe one person's experience — which, in turn, may not have significance for someone halfway around the world.
Source: Studies in the Psalms: Adult Teacher's Insights, page 80.