viewport viewbox

What is the difference between a Viewport and Viewbox?
I don't understand the difference from their definitions in the the SVG specifications.

viewBox is attribute only for svg elements, it define a viewport for svg element
Each element in svg has a viewport, this viewport depend from
- viewBox attribute for parent svg node
- transformations apply to parent group
- transformations apply to element
- also x y width end height values for use or svg subelements

Michel