SharePoint 2013 comes with two SharePoint workflow platforms (SharePoint 2010 Workflow platform and SharePoint 2013 workflow platform)
SharePoint 2010 Workflow engine is installed automatically with SharePoint 2013 and this engine allows the workflows were built using SharePoint 2010 to continue working on SharePoint 2013. With SharePoint 2010 workflow platform you will have the same experience with building workflows on SharePoint 2010.
You can have the features of SharePoint 2013 workflow platform when you install and configure Windows Azure Workflow service.
SharePoint 2013 workflow platform includes these new features:
Workflows built using Windows Azure workflow can take advantage of several new capabilities
Note: this article is based on SharePoint 2013 Preview and there might be some changes on the release version