Large(r) companies tend to have a defined process for implementing change in their data centers, but small(er) companies tend not to. (Your Mileage May Vary.) This makes sense, as large companies usually have more systems dependent on each other, and a change made to one system can dramatically impact the effectiveness of another system. Small(er) shops tend to feel that those implementing the changes know the impact of the change they're making.
What is change management? Well, it's a well-defined process to document changes made to your systems. The changes include hardware, software, network infrastructure, configuration settings, or anything that can disrupt the normal processing of your business applications. The process usually includes a description of what change will be made, who will make the change, when it will be made and (most importantly) what steps will be taken if the changes were unsuccessful. The changes are generally reviewed by people representing groups who will be impacted by the change. These people often have the responsibility to approve the proposed changes.
The documentation of the changes provides a great reference tool for those times when problems occur and are traced to some change event, because it's then known when the change was made, by whom, and the anticipated effects of the change.
I think the most important aspect of the change management process is the backout plan should the change not be successful. In too many cases the processes necessary to reverse changes aren't researched until after problems occur, when the change is disrupting business and there's significant pressure on those making the changes to "get it fixed". Sufficient time spent ahead of the change documenting what steps are required to back the change out can minimize the downtime and minimize that pressure.
(This is why flight training focuses so much on emergency procedures - keeping a level head when problems occur can truly save lives.)
If you haven't implemented a comprehensive change management process it's well worth your time.