What's the difference between fixed lens and varifocal lens cameras?

fixed lens camera, is a camera lens with a fixed lens which you can't adjust the focal length, angle of view and also level of zoom. but almost of all fixed lens cameras are wide angle and also smaller than varifocal cameras.

varifocal lens cameras,  is a camera lens with variable focal length in which focus changes as focal length (and magnification) changes, as compared to parfocal ("true") zoom lens, which remains in focus as the lens zooms (focal length and magnification change). Many so-called "zoom" lenses, particularly in the case of fixed lens cameras, are actually varifocal lenses. Usually the varifocal lens camera is 2.8-12mm, you can adjust it as you need. 

By increasing the focal length, you get a much better image of the man throwing the rock. At 10 feet, this doesn't matter very much but at larger distances this can mean the difference between identifying a suspect and not getting a clear enough picture of their face.

A varifocal camera allows you to adjust between its top and bottom limits on its view angle. This means that you have the ability to manually set both the view angle and level of zoom. Have a scenario where you need to zoom in on a gate or other access point? Use the 12mm setting (12 degrees and about 4x zoom). Need a wide angle view? Go with 2.8 mm (109 degrees). Need something in between? Try 6mm (~45 degrees) or 9mm (~30 degrees).

Normally the varifocal security camera is from 2.8 to 12 millimeters, this means that you can increase and decrease the degree of vision. normally the varifocal security cameras are more expensive than the fixed lens cameras, the lenses Fixed are the cameras that the angle of vision does not vary.