Flask was started around 2010, leveraging the Werkzeug WSGI toolkit (http://werkzeug.pocoo.org/), which provides the foundations for interacting with HTTP requests via the WSGI protocol, and various tools such as a routing system.
Werkzeug is equivalent to Paste, which provided similar features. The Pylons project (http://pylonsproject.org), which is the umbrella organization for projects like Pyramid --another web framework-- integrated Paste and its various components at some point.
Together with Bottle (http://bottlepy.org/) and a handful of other projects, they composed the Python microframeworks ecosystem.
All those projects have a similar goal--they want to offer to the Python community simple tools to build web applications ...
