What is the difference between a zoom lens and a varifocal lens?

A zoom lens maintains focus when changing field of view, whereas a varifocal lens does not necessarily maintain focus when changing the focal length. This means that with a zoom lens, you can zoom in or out and the focus will remain sharp, which is particularly useful for applications requiring continuous focus adjustments. In contrast, varifocal lenses need to be refocused after adjusting the focal length. This distinction is important in video surveillance to ensure clear image capture at varying distances.