At the most basic level, enzymes are classified into one of 6 categories (oxidoreductase, transferase, hydrolase, lyase, isomerase, and ligase) based on the type of reaction they catalyze (see
http://en.wikipedia.org/wiki/EC_number for more information). Monoamine oxidase would be classified as an oxidoreductase because it catalyzes the oxidation of a primary amine to an aldehyde. It would not be classified as a hydrolase, because these enzymes catalyze hydrolysis/condensation reactions, not oxidation/reduction reactions.