Workflow vs Skill
A fixed sequence of steps, versus a packaged capability the model reaches for on its own
Workflow
A defined sequence: do this, then this, then this, in a specific order every time. A workflow can use one or more skills along the way, but the sequence itself, not any single capability, is what defines it.
VS
Skill
A packaged set of instructions the model loads on demand for one job. A skill changes how the current agent handles a task, it doesn't define a fixed multi-step sequence the way a workflow does.
Which one do you actually need
A skill teaches an agent how to do one thing well. A workflow defines the order several things happen in, and can call on one or more skills as it goes. Skill is an ingredient, workflow is the recipe that decides when each ingredient gets used.