A workflow model is an enterprise process digitized in the platform. Typical candidates to this digitization are documents validation, purchase orders, case processing, invoice management.
From a technical point of view, a workflow is an automated process that may involve human interactions. This process has a beginning, an end and a state persisted at any time between this beginning and this end. A workflow is represented by a graph of nodes. At each node the workflow designer specified what should happen. This is how the process is automated.
The workflow engine is in charge of taking a workflow model, instantiate it and run it all the way long from the beginning to the end. Workflow models are designed with Nuxeo Studio.