Process Builder: The Lightning Process Builder is a new workflow tool that helps you easily automate your business processes by providing a powerful and user-friendly visual representation of your process as you build it. Instead of doing this repetitive work manually, you can configure processes to do it automatically. The Process Builder’s simple and powerful design allows you to automate processes.
Above both tool don't have any interaction with end user they work behind the scene.
Flow:
Visual Workflow lets you automate business processes by building flows and distributing them to the right users or systems. A flow is an application that can execute logic, interact with the Salesforce database, call Apex classes, and collect data from users. You can build flows by using the Cloud Flow Designer.
kinldy find the information,
Process Builder:
The Lightning Process Builder is a new workflow tool that helps you easily automate your business processes by providing a powerful and user-friendly visual representation of your process as you build it. Instead of doing this repetitive work manually, you can configure processes to do it automatically. The Process Builder’s simple and powerful design allows you to automate processes.
You can watch introduction video here
https://www.youtube.com/watch?v=hDYstN-M30U
Above both tool don't have any interaction with end user they work behind the scene.
Flow:
Visual Workflow lets you automate business processes by building flows and distributing them to the right users or systems. A flow is an application that can execute logic, interact with the Salesforce database, call Apex classes, and collect data from users. You can build flows by using the Cloud Flow Designer.
You can watch introduction video here
https://www.youtube.com/watch?v=RpEDVjLNgUc
Hope this will help you,
Mark Best ANSWER if its clear.
Thanks
karthik
You can use the Process Builder to perform more actions than with workflow:
But the process builder doesn’t support outbound messages.
Workflow does only 4 actions
Please check below post for same issue
1) https://developer.salesforce.com/forums/?id=906F00000005FSdIAM
2) https://www.safaribooksonline.com/library/view/learning-salesforce-visual/9781785289835/ch05s02.html
Let us know if this will help you