What is difference between v-app-bar and v-toolbar in vuetify?

Per the Vuetify Migration Guide -- 'Migrating from v1.5.x to v2.0.x':

v-toolbar: All existing scrolling techniques and app functionality has been deprecated and moved to v-app-bar.

Thus, starting in Vuetify 2.0, v-app-bar is what you probably want to use at the top of most typical apps since you can do scrolling-related effects and designate v-app-bar as a unique 'top level' application component using the app prop.

On the other hand, v-toolbar could be used for other 'subordinate' cases where scrolling should definitely not affect the toolbar, perhaps if v-toolbar is being used as part of a in-screen widget.

Actually v-app-bar extends v-toolbar to give you additional properties that you can use.

These properties give you far more granular control over the overall layout of the toolbar and how it responds to sizing and content changes in the surrounding space.

You can look at the properties of each and see how the v-app-bar has a dozen or more additional properties that you can leverage to customize the functionality and design of it, whereas the toolbar is opinionated about its purpose and limits those functionalities.