Flow and Process Builder are both automation tools in Salesforce, but they have some key differences:

Purpose: Flows are used to automate business processes that span multiple objects and automate complex business processes, while Process Builder is used to automate simple and straightforward business processes that are based on one object.

Complexity: Flows are more complex and flexible than Process Builder, and provide more options for designing, building, and testing processes. Process Builder is simpler and easier to use for basic automation tasks.

User Interface: Flows have a graphical user interface that allows you to visually design and build processes, while Process Builder has a more straightforward and simplified user interface.

Functionality: Flows provide more advanced functionality, such as the ability to use record-triggered flows, create loops, and use variables, while Process Builder provides a more limited set of functionality but is easier to use.

In summary, if you need to automate complex business processes or if you are looking for more advanced functionality, you may want to use Flows. If you have simple and straightforward business processes, Process Builder may be the better choice for you.