A fretboard is a piece of wood usually constructed out of maple, rosewood, or ebony. This piece of wood is typically finely finished and fastened to the neck of a guitar or other fretted instrument with some sort of binding or glue. Frets are embedded into the wood with a slight distance between the top of the fret and the fretboard to allow for clearance of strings.
The most important part of the fretboard is that it be finely finished and smooth to allow for easy movement of the fingers up and down it while playing a fretted instrument.