A value stream is all the activities — value-adding and non-value-adding — required to bring a product or service from order to delivery. The value stream is the unit of analysis in Lean: this is where waste and flow become visible.
Most organisations are organised into departments, but value flows across them. That is why no single manager typically sees the entire value stream — and the problems arise precisely in the handovers between departments, where responsibility is unclear.
Value Stream Mapping (VSM) is the method for drawing the value stream: current state first, then future state and an action plan for getting there.