In SharePoint 2013, workflow has been decoupled from the SharePoint server and runs as the Workflow Manager Server in on-premises installations and as Workflow Manager Service in the cloud. The new workflow engine is written according to the new cloud app model architecture, leveraging the REST _API to call back into SharePoint via the OAuth protocol and employing Windows Azure Service Bus to provide its messaging framework and durable event management.
The tools the business analyst, SharePoint subject matter expert, and developer continue to use are Visio Professional, SharePoint Designer, and Visual Studio, respectively, but these tools sport a host of new features for building workflows. Visual Studio can be used to build custom actions that can be installed into SPD to provide additional productivity gains for those creating workflows.
Lastly, workflows fit very comfortably inside your Apps for SharePoint. Those who are skilled at developing workflows in Visual Studio can build workflows around very common business process scenarios, package them up as an app, and get them into the SharePoint Store on Office.com. Also, because these workflows are XAML-based, they can run equally well both on-premises and on Office 365.