Python's Flask micro web framework for Create Online Apps

Flask is a web framework that allows developers to build lightweight web applications quickly and easily with Flask Libraries. It was developed by Armin Ronacher, leader of the International Group of Python Enthusiasts(POCCO). It is basically based on the WSGI toolkit and Jinja2 templating engine. Flask is a microframework that does not include a database abstraction layer, form validation, or any other components where pre-existing third-party libraries provide common functions. It is designed to make getting started quick and easy, with the ability to scale up to complex applications.

Flask is built on top of the WSGI (Web Server Gateway Interface) toolkit and the Jinja2 templating engine. WSGI is a specification that enables web servers and web applications to communicate with each other. It defines a standard interface between web servers and web applications or frameworks, allowing developers to write web applications that can run on any WSGI-compliant web server. Flask uses WSGI to handle HTTP requests and responses. 

Jinja2 is a powerful templating engine for Python that enables developers to build dynamic web pages by embedding Python code in HTML templates. Flask integrates with Jinja2 to provide a simple and intuitive way to generate HTML pages dynamically. 

Flask uses Jinja2 templates to render views, which are the HTML pages that users see in their web browsers. Flask's relationship with WSGI and Jinja2 is that it uses both of these tools to provide a simple and flexible way to build web applications. Flask leverages the power of WSGI to handle HTTP requests and responses, and it uses Jinja2 to render dynamic HTML pages. By building on top of these established tools, Flask provides a solid foundation for web development that is both powerful and easy to use. 

Sure, here is a simple Flask application in Python: 

from flask import Flask

app = Flask(__name__)

@app.route("/") 
def hello():
        return "Hello, World!"

if __name__ == "__main__":
    app.run(debug=True)
              

This code creates a new Flask web server from the Flask module and the variable app. The @app.route("/") is a Python decorator that Flask provides to assign URLs in our app to functions easily. The `"/"