Web application with python

I have a Python program that takes one input (string) and prints out several lines as an output. I was hoping to get it online, so that the web page only has a text-area and a button. Inserting a string and pressing the button should send the string to python and make the original page print out the output from Python.

Assuming I have only dealt with Python and a little bit of HTML and have very limited knowledge about the web, how might I approach this problem?

For some reason, my first instinct was PHP, but Googling "PHP Python application" didn't really help.

That is, assuming what I want IS a 'web application', right? Not a web server, web page or something else?

How could I get a really simple version (like in the first paragraph) up and running, and where to learn more?