Here at the Worx, we often build Drupal websites as intranets. These have on many occasions, become full office automation centers and allow for faster work and better communications. But, they are also VERY custom and require a large degree of testing by the customer as we move towards production.
There are a variety of 'tests' that an application must go through before it becomes production. Most of this is done behind the scenes by the developers as they work their way through the development of your new application.
There are also a variety of 'tests' that only the client can perform. If the website being developed is more of an 'application' then just content for general consumption, the client responsibility goes up dramatically. Like any other application that is written, this needs extensive testing. This usually includes at least beta testing and often, parallel testing. In many cases, this is done one section at a time.
Beta testing really means that you are aware that errors WILL occur. This is the 'dry run' time where you try every function of the application multiple times. Does it do what is expected? Is it consistent? Is the user interface acceptable? All of this testing should be done with a notebook by your side and extensive notes taken to feed back to the developers any problems you may encounter.
Parallel testing is used whenever you are replacing an older application with something new. In this instance you will do all your processes TWICE. Once in each system. The basic question here is simple. Do you get the same results out of both systems? If not, is that what you expected? Part of the reason for the replacement might be improvement and correction. But in most of the application, you should get the same results or be able to explain why based on new functionality.
Any time an application is being replaced, it is absolutely imperative that a parallel test occur. To go directly into production with an untested application is suicide. If you take a new application and begin using it for production WITHOUT parallel, then you simply failed to test and will get problems. Usually lots of problems.
So, in summary… If you have a new application with new functionality, you will go through a Beta phase. If you are replacing an old application with a new application, then you will go through Beta AND Parallel. Don't skip the Parallel or you may never know what the problems are in the new application until it is much too late!
To truly enjoy your new application, remember that testing is a very important part of the client responsibility.