Pearl is a material that reflects light (like metallic) but also allows some of the light to pass though and reflect back from the surface below. The light reflected may be un-obstructed, or may pass through another particle of pearl, adding the illusion of depth, and altering the hue.
Paints containing pearls usually contain a higher resin to pigment ratio and require more coats of material to obtain full coverage.
The newer tri-coat systems involve a base coat, a clear/tint coat, and a final clear coat. This creates a very nice effect, but makes spot repair more difficult.