The Server Object Model will be executed in the server side & it provides rich set of classes in representing & manipulating SharePoint objects. Server object model is like production server environment to access the data where sharepoint installed on machine .
Server Object Model is the most extensive API set available for SharePoint 2013. The core assembly is Microsoft.SharePoint.dll which is installed in the Global Assembly Cache.
The general classes are available in the Microsoft.SharePoint namespace and the administration classes inside the Microsoft.SharePoint.Administration namespace.