A workflow is Copilot Studio's newer, AI-assisted way to automate a process. Like an agent flow, it starts on a trigger and runs at least one action. You build it in a visual drag-and-drop designer: describe the automation you want, and Copilot Studio helps generate, configure and validate it.
The difference is in the steps. In a workflow each step can reason and act, and a workflow can call an agent to complete a step. Built-in testing and human-in-the-loop steps are part of the designer. Workflows run on the GitHub Copilot harness, which bills Copilot Credits from the moment you start building, testing included.