Both types of lens enable us to change focal length and focus distance.
Zoom lenses keep the focus when we change the focal length. Vario lenses generally don’t.
The change of focal length is usually achieved by changing the relative position of lens packages. During this process, the principal planes move.
For a f=6-18mm lens with the focus distance set to infinity the principal plane of f=6mm must be 6mm and at f=18mm 18mm from of the image plane respectively,i.e. from the sensor. Therefore, when the lens’ focal length is changed from 6 to 18mm it must move by 12mm.
For zoom lenses this adjustment is done with the very same Ringthat is used for the change of focal length. Vario lenses use a different adjustment ring for this.
see also: How to tell which sensor size a zoom lens was designed for?
see also: I can’t focus my lens to infinity#This lens doesn’t focus, but another zoom lens does