WSGI is the Web Server Gateway Interface. It is a specification that describes how a web server communicates with web applications, and how web applications can be chained together to process one request. WSGI is a Python standard described in detail in PEP 3333.
In a single sentence, WSGI is just an interface specification by which the server and application communicate. This interface used by many popular frameworks like Flask and Django is defined in the PEP 3333 specification for both parties: server and WSGI application.
Python 2.5 and later comes with a WSGI server which will be used in this tutorial. In 2.4 and earlier it can be installed.
A short-list with web frameworks with native WSGI support:
Flask - Flask is a microframework for Python based on Werkzeug, Jinja 2 and good intentions
Django - The web framework for perfectionists with deadlines.
Pyramid - Pyramid is a minimalist web framework aiming at composability and making developers paying only for what they use.
Falcon - Falcon is a high-performance Python framework for building cloud APIs
WSGI - official documentation
An introduction to WSGI by Titus Brown
Frameworks that run on WSGI - a curated list