"Start the database!" is an incorrect phrase. Back to basics with databases and instances

On StackOverflow, I chanced upon a beginner being trolled as they struggled to get going with Oracle. "Duh! Just start the database" someone said dismissively. I called out the rudeness by replying: "You don't start a database, you start an instance". So lets go back to basics, and describe Oracle in its most fundamental terms: What is a database? What is an instance? And why its important to understand the difference.  Catch me on Twitter at https://twitter.com/connor_mc_d