The Value Stream represents the work involved in the flow of Business value from initial idea to realization of value by the customer. It includes Business Discovery (prioritization, planning, staging, getting ready to pull) and Business Development (iterative development and incremental release).
In Lean-Agile, there are three levels in the value stream: Portfolio, Program, and Team.
Resources related to value stream levels
How Easily Can We Import a Lean or Agile Framework Into the Organization? (Blog Entry)