sources:usability.gov & smashingmagazine.com
User Testing is about having a user review a webpage and evaluate it, so that before the page is pushed out to everyone they have gotten the feedback of the public that they have surveyed. They receive data, and feedback from the users that have been selected to evaluate that product. As a coder after every step you should have a user testing, to make sure that what you adiuence is viewing is what you are going for.
The different ways in which people are able to do a user test is by using the application, just look at the application but not play around with it, or they can use the application and afterwards answer some questions and talk with other members who also did the application.
Different versions of browsers are tested by using the different applications available to access the website. Whether it's using a phone, compared to a large screen, compared to an average screen computer. There are also websites out there that provide you the different application views out there, so that way you(as the coder) are able to see what the different screens look like based on the size.