A parfocal lens is a zoom lens that holds its focus throughout its entire zoom range. A varifocal lens, on the other hand, is a zoom lens that requires the lens to be refocused if the focal length is changed.
Parfocal lenses offer photographers a few conveniences over varifocal lenses. Because parfocal lenses can be zoomed without needing to be focused, a photographer can take pictures at different focal lengths more quickly. These lenses also help photographers focus more easily because they can be zoomed to their maximum focal length to focus and then zoomed to a shorter focal length to compose a picture.
Parfocal lenses also provide benefits when used for motion pictures; they allow a person making a movie or video to zoom in or out while keeping an actor in focus. Using varifocal lenses for this application would require stopping the actors in the middle of their dialogue to adjust the focus of the lens or cause the image to go out of focus, which could distract the audience.
Today, particularly in fixed-lens cameras, many zooms are actually varifocal lenses that act like parfocal lenses. Varifocal lenses are easier to make than parfocal lenses. Autofocus and processing within the camera, however, allow focus to be adjusted automatically during zooming to make its operation similar to a parfocal zoom.