Django follows Python's philosphy of "batteries included". In particular, it uses its own ORM. It also has a large ecosystem of pluggable apps, whether contrib modules or outside the Django tree.
Flask is a microframework. It lets you build your website in a single file if that's what you need, or scale up to multiple files/sub-websites if your website is more complex. It doesn't have its own ORM, but it's very easy to use SQLAlchemy (the best ORM/SQL abstraction framework out there, IMHO). Its templating engine (Jinja2) is superior to Django's. On the other hand, the list of plugins available is smaller than Django's.
Flask is a microframework. It lets you build your website in a single file if that's what you need, or scale up to multiple files/sub-websites if your website is more complex. It doesn't have its own ORM, but it's very easy to use SQLAlchemy (the best ORM/SQL abstraction framework out there, IMHO). Its templating engine (Jinja2) is superior to Django's. On the other hand, the list of plugins available is smaller than Django's.
Both have very good documentation.