Flask is a python microframework. Aimed mainly at small applications and quick development.
Ranked in these QuestionsQuestion Ranking
Pro Minimalist without losing power
Flask is very easy to get up and going, with vanilla HTML or with bootstrap pieces. It doesn't take much lines of python to load Flask to get headers working, etc, and since it's all modular you don't have to have something you don't want in your application. Each module within Flask works very well, and is extremely good on bandwidth and CPU usage.
Pro Very flexible
The choice of not having an ORM, but instead choosing one suited to the task, gives developers who use Flask a great deal of flexibility. Another area where Flask gives a lot of options to developers is the templating. They can use Jinja2, Flask's default templating language or choose from a number of different templating languages they desire.
Pro Simple yet powerful
Built on top of Werkzeug and Jinja 2, Flask is able to continue with their designs of being simple and powerful tools for web development.
Pro Simple to use
With only a few lines of code, you can have a very simple site up and running.
Pro Many community extensions
Con Not async-friendly
Flask is explicitly not designed to handle async programming.