I think you are looking at this.... Database servers—Help | ArcGIS for Desktop
This is a completely separate product that is not required to use ArcGIS for Desktop. I would not use it unless needed, too many limitations in my opinion.
Here is more details on database servers: Types of geodatabases—ArcGIS Help | ArcGIS for Desktop - Look at the section - How is ArcSDE technology included in ArcGIS?
Hope this helps!