I am working on a  web application, in which clicking on some link another popup windows appears. The pop windows is not an alert but its a form with various fields to be entered by user and click "Next". 
How can I handle/automate this popup windows using selenium.
Summary :-

Click on the hyperlink (url) - "CLICK HERE"
A user registration form appears as a pop up
A data is to be filled by user 
Click Next/Submit button.
Another next redirected to another page/form 'User Personal Information Page'
Personal information is to be filled by user
Click "Next/Submit"
Popup disappeared.
Now further processing on original/Base page.
If its a new window or an iframe you should use the driver.switch_to_frame(webelement) or driver.switch_to_window(window_name). This should then allow you to interact with the elements within the popup.
After you've finished, you should then driver.switch_to_default_content() to return to the main webpage.
Python webdriver to handle pop up browser windows which is not an alert