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:
Code generation of strong types provided by customizations to the code generation tool (CrmSvcUtil.exe)
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.