Workflows are limited to only 4 actions
Create Task
Update Fiel
Email Alert
Outbound Message
Process Builder does all that and more, with the exception of Outbound Messages. I found this trailhead helpful in understanding process and flow builders.
For updating related records, Process Builder can update any field on any related record, where Workflow can only update some fields on a parent record of a Master-Detail relationship. … Also,Process Builder finally gives admins the ability to set the exact order of operations, whereas with Workflow we had no control.