setEnabled() vs setClickable(), what is the difference?

What the hell is that mean?

Quoting the Wikipedia page for "GUI widget":

In the context of an application, a widget may be enabled or disabled at a given point in time. An enabled widget has the capacity to respond to events, such as keystrokes or mouse actions. A widget that cannot respond to such events is considered disabled. The appearance of disabled widget is typically different from an enabled widget; the disabled widget may be drawn in a lighter color, or may be visually obscured in some way. See the image to the right for an example.

This concept has been around for a couple of decades and can be found in most GUI frameworks.

what is the difference between enable state/clickable state and disabled state/ not clickable state?

In Android, a widget that is not clickable will not respond to click events. A disabled widget not only is not clickable, but it also visually indicates that it is disabled.

what do you mean by: "..since it makes the Button visually "disabled"? how does it changes it visually?

What makes a Button look and respond like a Button is its background, which is a StateListDrawable. There is a specific image used for the disabled state.

A big difference I don't see mentioned elsewhere is with overlapping Views. A View with clickable=true and enabled=false won't allow you to press a View behind it. But a View with clickable=false will allow you to press a View behind it.

So basically an enabled false doesn't respond to any response and an clickable false still response when set at runtime and trust me I just tried it.