A workflow instance is a running instance of a workflow definition.
Once a workflow instance has been started, it can not be changed. If you change the
underlying process definition, it will be versioned. Any new workflow instance will reflect any
changes to the workflow definition. Any old instances currently running will reference the old
definition.
Workflow instances survive server restarts, so all user tasks will still be running if you stop and restart the server. Process and task execution variables also survive server restarts.