Raku is a ceramic firing process where the piece is heated very fast in a kiln to 1750 - 1900 degrees.  
Once the glaze is mature the piece is pulled from the kiln while red hot and placed in a container with a combustible like paper.
The chemicals in the glaze mix with the fire and smoke providing the colors on the finished piece.  
I limit my Raku glaze palette to a few glazes but the outcome is completely different dependent on the process I use to glaze and fire my piece.  Because of this process no two pieces are the same, even when the same glaze is used.