Webpage that updates with user input?

I have an idea for something I would like to work on -- but I'm not too sure what exactly it is that I should be searching for. Can anyone advise?

I would like to create a webpage that allows users to input select data via buttons on the page. The page will automatically update so that any user currently on the page will see the latest change.

a webpage that:
-accepts user input
-updates page with input
-updates all open sessions

Thank you