"Originally modeled on the Psalms and other poetic passages (commonly referred to as "canticles") in the Scriptures, Christian hymns are generally directed as praise and worship to the monotheistic God. Many refer to Jesus Christ either directly or indirectly. Since the earliest times, Christians have sung "psalms and hymns and spiritual songs", both in private devotions and in corporate worship (Matthew 26:30; 1 Cor 14:26; Ephesians 5:19; Colossians 3:16; James 5:13; cf. Revelation 5:8-10; Revelation 14:1-5). One definition of a hymn is "...a lyric poem, reverently and devotionally conceived, which is designed to be sung and which expresses the worshipper's attitude toward God or God's purposes in human life. It should be simple and metrical in form, genuinely emotional, poetic and literary in style, spiritual in quality, and in its ideas so direct and so immediately apparent as to unify a congregation while singing it."
(Summary by Wikipedia)