An Enzyme is usually a protein but can also be an RNA which functions as a catalyst and speeds up a process. It lowers the energy needed for a reaction to happen and makes the reaction go faster than it would have naturally. An enzyme usually doesn't get changed by the process itself but can be used again. This is not the case for all ribozymes, which are RNA enzymes, because some of them cut themselves up. But some of them are true enzymes, left unchanged by the process and ready for new reactions.