In practice
You start the interaction. The assistant answers, then stops. It doesn't watch a data source or act on a schedule unless another system tells it to.
That's different from an agent. An agent gets a goal, takes several steps and decides what to do next along the way.
Why it matters for product builders
An assistant can save time on a task, but someone still has to prompt it. That's fine for occasional work. It gets expensive when the work must happen continuously.