Read this before you get started with Flask. This hopefully answers some questions about the intention of the project, what it aims at and when you should or should not be using it.
The micro in microframework for me means on the one hand being small in size and complexity but on the other hand also that the complexity of the applications that are written with these frameworks do not exceed a certain size. A microframework like Flask sacrifices a few things in order to be approachable and to be as concise as possible.
For example Flask uses thread local objects internally so that you don't have to pass objects around from function to function within a request in order to stay threadsafe. While this is a really easy approach and saves you a lot of time, it also does not scale well to large applications. It's especially painful for more complex unittests and when you suddenly have to deal with code being executed outside of the context of a request (for example if you have cronjobs).
Flask provides some tools to deal with the downsides of this approach but the core problem of this approach obviously stays. It is also based on convention over configuration which means that a lot of things are preconfigured in Flask and will work well for smaller applications but not so much for larger ones (where and how it looks for templates, static files etc.)
But don't worry if your application suddenly grows larger than it was initially and you're afraid Flask might not grow with it. Even with larger frameworks you sooner or later will find out that you need something the framework just cannot do for you without modification. If you are ever in that situation, check out the :ref:`becomingbig` chapter.
Flask is not only a microframework, it is also an example. Based on Flask, there will be a series of blog posts that explain how to create a framework. Flask itself is just one way to implement a framework on top of existing libraries. Unlike many other microframeworks Flask does not try to implement anything on its own, it reuses existing code.
I'm not even joking. Well, maybe a little. If you write a web application you are probably allowing users to register and leave their data on your server. The users are entrusting you with data. And even if you are the only user that might leave data in your application, you still want that data to be stored in a secure manner.
Unfortunately there are many ways security of a web application can be compromised. Flask protects you against one of the most common security problems of modern web applications: cross site scripting (XSS). Unless you deliberately mark insecure HTML as secure Flask (and the underlying Jinja2 template engine) have you covered. But there are many more ways to cause security problems.
Whenever something is dangerous where you have to watch out, the documentation will tell you so. Some of the security concerns of web development are far more complex than one might think and often we all end up in situations where we think "well, this is just far fetched, how could that possibly be exploited" and then an intelligent guy comes along and figures a way out to exploit that application. And don't think, your application is not important enough for hackers to take notice. Depending on the kind of attack, chances are there are automated botnets out there trying to figure out how to fill your database with viagra advertisements.
So always keep that in mind when doing web development.
Is Flask for you? If your application small-ish and does not depend on too complex database structures, Flask is the Framework for you. It was designed from the ground up to be easy to use, based on established principles, good intentions and on top of two established libraries in widespread usage. Recent versions of Flask scale nicely within reasonable bounds and if you grow larger, you won't have any troubles adjusting Flask for your new application size.
If you suddenly discover that your application grows larger than originally intended, head over to the :ref:`becomingbig` section to see some possible solutions for larger applications.
Satisfied? Then head over to the :ref:`installation`.