Simply put, fusing is the joining together of glass usually in a kiln at a high temperature around 700c (tack fusing) to 825c (full fusing). Using this range of temperatures can alter the effect of the glass to either retain some of the texture or to merge the glass flat, often to 6mm.
When glass is fused together whether it be strip sheets, powder or small broken pieces of different sizes (called frit) they must be compatible with each other (having the same Coefficient Of Expansion). If glass is used of different COE and fused together, this is likely to crack/break during creation or even in the near future due to stress.
The majority of my work uses a brand called Bullseye which is one of the leading suppliers of raw glass materials which ensures their products are factory-tested for compatibility with other Bullseye glass.