You could see extensions as web applications included on your QlikView apps. They start running on the client's side and restart after every action you do on your QlikView app (change tab, filter, selection...). They can connect to the data of your QlikView app to display visualizations. As they run in your browser, make sure that the extensions have been tested for different browser compatibility.
I recommend that you check out this document:
Also, if you are interested in developing extensions, check out this template (it can help a lot):
Here is the SDK just in case: QlikView Version 11 SDK