The test site
If you want to experiment with Drupal by installing additional modules, a test site provides a safe place to do this. For all of the obvious reasons, your live class site is not the place to experiment or take chances. Although installing a test site is additional work, it provides you with a safe place to learn and experiment without fear of consequences.
Fortunately, the process of verifying your backup, as described in the previous section, also gets you your test site.
The test site is the site that you should use when you are trying something new, from evaluating a new module or theme, to testing an upgrade procedure. Using the test site allows you to take chances you would not otherwise be able to take. For example, if you want to try a new module, you can install it and experiment with the functionality and settings on your test site. If the module does what you need, then you can deploy it on your live site. If, however, the module does not meet your needs, you can just...