A varifocal lens is a non-fixed focal length lens where the focus changes with focal length. This is in contrast to true zoom lenses, which retain correct focus throughout their focal length range. True zooms have a constant maximum aperture at all focal lengths (as in a 28-70mm f/2.8 lens), while varifocals have maximum apertures that increase (in number, but decrease is size!) as the focal length increases (as in a 28-70mm f/3.5-5.6 lens, which has a maximum aperture of f/3.5 at 28mm, that becomes smaller in size as the focal length increases until it reaches a value of f/5.6 at 70mm).
Varifocals are easier to design and build than true zooms which explains their ubiquity in the camera market. Note that they are commonly, but erroneously, referred to as zoom lenses by users and manufacturers alike.