Question

what is the difference between focus and principal focus?

Answer 1

Principal focus is a point (on the principal axis) where parallel incident rays meet after reflecting (in mirrors) or refracting (in lenses). It is a fixed point of particular lens/mirror and depends upon its shape. Principal Focus also determines the focal length of the lens/mirror. 

Focus is just a random point where some rays meet, it might be at the principal focus or otherwise. It also need no be placed on the principal axis of the lens/mirror.

Answer 2

Principal focus, it turns out that if a beam of light , all parallel to principal axis of lens, falls on the lens, they either all converge to a point in its principal axis.
Focus, when a beam of light appears on particular point countinuously, it is the focus.