Symbolism, Imagery, Allegory
Every musician needs a trademark. Justin Bieber has (had!) his helmet hair, Kanye's got his "shutter shades", and Orpheus has his rockin' lyre. But, uh, what exactly is a lyre?
Basically, a lyre is a harp that's been hit by a shrink ray. It has approximately seven strings, is small enough to carry around with you, and plays really pretty music. Back in the day, people strummed on lyres while reciting long, emotional poems. The word "lyrical" (meaning "to express words in a beautiful way") has its roots in "lyre," thanks to the instrument's association with poetry. Tell that one to your English teacher.
In general, the lyre symbolizes an appreciation for music and the arts. Gods and mortals who carry lyres tend to be more on the sensitive side. If you ever see an image of a handsome-looking dude hauling around this small string instrument, chances are it's either Apollo or Orpheus, who are two of the most musically-inclined figures in Greek mythology.
In most paintings, the lyre is portrayed as a small wooden instrument. But sometimes the gods would get creative and make lyres out of tortoise shells. For example, when Hermes stole Apollo's sheep, he crafted a lyre out of a turtle shell in order to appease the music-loving god. Actually, this is rumored to be the first lyre ever made. (Hermes was crafty like that.)
Interestingly, the lyre is not exclusively Greek. Although it went by different names, the lyre also appeared throughout Europe, Scandinavia, and the Middle East.