Flask or Django?

Both are mature Python web frameworks that enable you to build any kind of web apps.

Flask is microframework with small api, very easy and straightforward to learn, perfect for small projects, Flask's official documentation is quite good. In comparison, Django is a much larger more complex framework bundled with lots of stuffs & admin interface, the app you write with Django should follow the MVC (or MVT whatever the Django community likes to call it) architecture. Django has a much larger community and more books written on the subject than Flask.

If you're a beginner without much prior back-end development experience, Flask is probably a better choice to go for as it's more beginner-friendly. Chances are if your ultimate goal is to become a Python web developer, you'd probably endup learning both frameworks one way or the other, and knowing Flask should make learning Django much easier.