What is the difference between an 230V/120V AC automatic voltage regulator and a step up / step down voltage transformer / converter?
An automatic voltage regulator / constant voltage transformer / ferro-resonant transformer would be a special transformer that uses nonlinear magnetic properties and a resonant circuit to provide a stable output voltage over a wide range of input voltage. It could be either a 1:1 or a step-up / step-down transformer. 

A step-up / step-down transformer would be a standard transformer performing only that function.