What are enzymes in the digestive system?

I have learnt about the digestive system in school but I was still unsure about the part on enzymes so i went online and read a little on it. I found out that digestive enzymes are enzymes that break down food into their simpler substances, in order to facilitate their absorption by the body. Digestive enzymes are diverse and are found in the saliva secreted by the salivary glands, hydrul in the stomach secreted by cells lining the stomach, in the pancreatic juice secreted by pancreatic exocrine cells, and in the intestinal (small and large) secretions, or as part of the lining of the gastrointestinal tract. 

Digestive enzymes are classified based on their target substrates:

• proteases and peptidases split proteins into amino acids.
• lipases split fat in three fatty acids and a glycerol molecule.
• carbohydrases split carbohydrates such as starch and sugars into simple sugars such as glucose, the simplest sugar on earth.
• nucleases split nucleic acids into nucleotides.