What is Process Builder?
Process Builder is a modern way to implement state machines using the If This Then That (IFTTT) paradigm, with a rich user interface that allows any devmin to exactly understand how the automated process is shaped at a glance.
Just as we did with Workflow Rules, select a Salesforce object, define the trigger criteria, and set up the actions you want to execute in order to create a brand new automation.
Unlike Workflow Rules, you can define the order in which actions are executed, while in workflows, you cannot predict which action will be executed first.
Another difference is that Process Builder can deliver automation not only for Salesforce objects but also for Platform Events (which are a special kind of Salesforce object used to send events back and forth inside and outside the platform that we won't cover in this book).
Further reading
For more info about Platform Events, have a look at the Platform Events Basics Trailhead module at...