Creating a Drupal site can be a complex process. Some people put together Drupal sites using a collection of different modules whilst others use Drupal as a framework and build the site using code.
No matter what sort of Drupal site you have, you'll need to have some testing in place to make sure that it works correctly. This is especially important when applying updates to your sites as the updated code can create unwanted side effects (or contain bugs) that might cause the site to malfunction. Generally, Drupal modules are very good with updates, but there are sometimes interactions between modules can cause problems that weren't taken into account.
When it comes to testing, there are a number of different options available to a Drupal developer. Each of which have their own uses and depend on the type of Drupal site you are trying to test.
In this article, I will go through a number of testing strategies that you can use to test a Drupal site. I won't delve too deep into each topic as this is more of an overview of the different technologies and strategies available.