Developer Extensions is a set of tools provided in the SDk that intendes to simplify and accelerate development. Usually to be used in Client apps, something like Portals.
It provide the following capabilities:
Simplified connection to CRM servers provided by the CrmConnection class (Microsoft.Xrm.Client)
Code generation of strong types provided by customizations to the code generation tool (CrmSvcUtil.exe)
App.config and Web.config configurability for enabling custom extensions provided by the CrmConfigurationManager class (Microsoft.Xrm.Client)
Performance enhancements through caching of service results provided by the CachedOrganizationService class (Microsoft.Xrm.Client)
Don’t forget:
Developer Extensions for Microsoft Dynamics CRM supports deployment for CRM on-premises and CRM Online. However, plug-ins that run in the sandbox on CRM Online shouldn’t make use of any classes or methods in Microsoft.Xrm.Client because certain functionality in that assembly isn’t supported in the sandbox.