Flask is a micro framework for Python web development. A framework, in the simplest terms, is a library or collection of libraries that aims to solve a part of a generic problem instead of a complete specific one. When building web applications, there are some problems that will always need to be solved, such as routing from URLs to resources, inserting dynamic data into HTML, and interacting with an end user.

Flask is a micro framework because it implements only core functionality (including routing) but leaves more advanced functionality (including authentication and database ORMs) to extensions. The result of this is less initial setup for the first-time user and more choice and flexibility for the experienced user. This is in contrast with "fuller" frameworks, such as Django, which dictate their own ORM and authentication technologies.

As we'll discuss, our Hello World application in Flask can be written in only seven lines of code, with the entire application consisting of a single file. Does that sound good? Let's get going!