As a result of its involvement in a reaction, an enzyme is unchanged.
Enzymes are catalysts that speed up chemical reactions that occur within the biological systems (they increase the reaction rate by lowering its activation energy). Enzymes react with substrate that binds to them (key-lock mechanism) and convert the substrate into the product. Most of enzymes are proteins by their structure but a few are catalytic RNA molecules-ribosymes.
Enzyme activity can be affected by other molecules (activators and repressors), pH, temperature etc.
Cellulase is an enzyme. Like other enzymes cellulase is mostly made of proteins. Cellulase is used to catalyze (digest) celluloysis, which is a polysaccharide in plants' cell wall.
