1. Amazon.com stores data about products, availability, customers, customer reviews, advertisements,etc.
   Ebay.com also stores data about products, customers, customer reviews about the sellers, etc.
3. Course-many, classroom-many, professor-1, department-many
4. Tracks, artist, release year, producer, etc.
5. Query results just show what you are looking for.  It does not show all the information on the table.
   Not stored in a database either.