Value stream mapping is a tool that helps visualise how work flows, not what work is flowing. 
It shows the major steps from end to end in the flow of value to a customer, along the value stream. Like other business maps, value stream maps help you better understand your organisation, but value streams take a much larger, more holistic view than conventional process mapping, looking at the value produced, not internal activity. They analyse macro workflows to identify waste and inefficiencies.
Note that there is an assumption here that work moves at least approximately in a linear stream from start to finish. This is often true in manufacturing but not so true in other sectors where it is more of a messy multi-directional network of value. So be sure the model is applicable before you try to draw it.
A Value Stream Map can have three streams mapped:
- It always depicts the flow of a work item
- It often depicts the flow of information along with that work item
- It occasionally maps the flow of materials to create the work item
It is important to do value stream mapping early, to get the big picture. This informs so much of our thinking about how to control and improve flow of value to the customer. Only once we begin to understand the system can we make valuable improvements.
The resulting square-wave timeline – of when we are doing value work on the item and when the work item is idle – is always enlightening. The ratio of the total of this value time to the total lead (elapsed) time is the efficiency of the flow. People are often horrified at how low it is, how much time that items of work spend lying around
For more, see our book Working Naked
See also Management in VSM