title: What is the difference between SQLAlchemy, PostgreSQL, and SQLite?
body: <p>I don't quite understand the difference between these three and I see SQLite and PostgreSQL within the SQLAlchemy - so are they a part of SQLAlchemy. Please clarify the differences and the relationships that these database systems may or may not share.</p>
  
  <p>Also, I'm building my app on Heroku - does Heroku support all three of them?</p>