Flask uses extensions in order to integrate transparently with third party libraries. WTForms with Flask-WTF is a good example of that as we will soon see. And, by the way, a Flask extension is a piece of code that integrates its configuration, context, and usage with Flask in a predictable way. That means extension usage is pretty similar. Now make sure Flask-WTF is installed in your virtual environment before continuing:

# oh god, so hard... not!
pip flask-wtf

From http://flask-wtf.readthedocs.org/, the project website, we have the following list of features offered by Flask-WTF:

We'll see the first two features in this chapter while the third will be discussed in Chapter 10, What Now?. The last three features will not be covered in this book. We advise you to explore them as homework.