What are enzymes?

What are enzymes? How do enzymes work? How does digestion work? How do I choose a product? Are enzymes safe? Can enzymes be an alternative to special diets? What is the recommended dose for enzymes? What is the best time for giving enzymes? Can I take enzymes with other medications or supplements? How do I store enzymes? I have never used an enzyme product. What can I expect as far as reactions? Will taking oral enzymes long-term stop the pancreas from functioning?