A zoom lens maintains focus when changing focal length. This is done with multiple groups moving to keep the back-focus of the image constant as the focal length changes. Most zoom lenses these days are variable aperture, simplifies the design. Constant aperture zooms tend to be big.
The Variable-Focal length does not do this- trades that convenience to provide a fast aperture, constant F2.8. But- means manually correcting the focus.
The ultimate cult Series I.