What's the difference between lightning flow and flow builder?

Process Builder:
Process Builder is a point-and-click tool that lets you easily automate if/then business processes and see a graphical representation of your process as you build.
Every process consists of a trigger, at least one criteria node, and at least one action. You can configure immediate actions or schedule actions to be executed at a specific time.
I suggest you, to complete the trailhead of process builder it has a better example to make you clear about it.
Flow:
Flow is a powerful business automation tool that can manipulate data in Salesforce in a variety of ways. Such an application can be created right from the org’s setup with just drag-drop/point-click. The ease of creating flows makes it the number one go-to tool when it comes to complex business requirements