Cupid's bow is actually a technical term for the shape of the upper edge of the upper lip. Theoretically, a properly formed upper lip is made of three distinct curves, which should look something like the curves of a bow.
You should have two soft but distinct ridges coming down from the sides of your columella (the bit of flesh that separates your nostrils). Between these ridges is a shallow depression. This formation is called the philtrum1.
Where the philtrum meets the upper lip, the curve of the lip is inverted. For about a centimeter, the lip curves upwards rather than downwards. From this center 'dip' the upper lip (when relaxed) curves down on either side of the philtrum until it meets the lower lip.
When performing reconstructive surgery on the lip (usually due to a congenital cleft lip or cleft palate), plastic surgeons pay special attention to reconstructing the philtrum and cupid's bow as earmarks of a successful and aesthetically pleasing reconstruction.
Lack of an easily distinguishable cupid's bow is one of the visible symptoms of fetal alcohol syndrome.
Cupid's bow is also sometimes used to describe other anatomical features that conform to the same shape. The bottom two lumbar vertebrae, for example, should also have perfect cupid's bow shapes on their inferior endplates. They are described as being "aimed cephalad" (aimed towards the head), with the arms of the bow pointing downwards, the same as they are in the upper lip.
1. From the Greek philtron, from philein, meaning 'to kiss'. Even anatomists recognize that this is a romantic area of the body. On a less romantic note, the philtrum was formed during embryonic development, when the nasomedial and maxillary process met and fused.