Complex sentence: A By-product is a secondary product deiving from a manufacturing process, chemical reaction, and is not the primary proct or service being produced.A by-product can be useful and marketable or it can be considered waste.
Split into simpler sentences: A by-product is a secondary product derived from a manufacturing process or chemical reaction. It is not the primary product or service being produced.A by-product can be useful and marketable or it can be considered waste.