SharePoint 2010 Workflow platform has been carried forward to SharePoint server 2013.
Initially, the Workflow Service in SharePoint 2013 is not connected for it requires a compatible workflow service such as Workflow Manager, and the SharePoint Designer 2013 has only SharePoint 2010 Workflow platform option.
The SharePoint 2013 Workflow platform only becomes available to you, and your tool, after you download and install the new workflow manager service and configure it to communicate with your SharePoint Server 2013 farm.
For guide on this process, see How to configure SharePoint2013 manager workflow.
Architecture changes in SharePoint workflow:
See more at: http://technet.microsoft.com/en-us/library/jj219638.aspx.