Glass fusing is the process of using a kiln to join together pieces of glass. If you apply heat to glass, it will soften.  If you continue to apply heat, the glass will become more fluid and flow together.  Two or more pieces of glass will stick (or "fuse") to each other.  You can also add colour pieces or frit to the glass which will also fuse. When the right kind of glass is heated and then cooled properly, the resulting fused glass piece will be solid and unbroken.