Bard songs are certain skills and spells which benefit from certain other
bardic abilities such as speedsong, and which are excluded from certain
abilities granted by other guilds, such as quickchant.

Most significantly, any hermetic spell with a verbal component which is
cast by a Bard wielding a lute is automatically a bard song.  This is
indicated by the message "You play the song." when casting.  This applies
regardless of the catagory or guild of the spell:  Magic Missile cast with
a lute is a bard song, and Sonic Blast cast without a lute, is not a bard
song.

Skills in the Bardic category are also bard songs.

Spells cast as bard songs are affected by the skills Speedsong, Singing,
and Playing, but not by Quickchant or any of the 'mancy' skills available
in mage tertiary guilds.

All bard songs are affected by the skills Scales and Centering.