Inlays are an optional design imbedded into the wood of an instrument. Inlays are often placed within the fretboard, around the Sound Hole, and other various places.
Inlays are often Opal or pearl. Fretboard inlays are often placed on frets 3, 5, 7, 9, and 12.
Some Luthiers will make elaborate designs throughout guitars using inlays, Dean guitars offers many acoustic guitars decorated with inlays.