Difference between OnClick() event and OnClickListener?

OnClickListener is the interface you need to implement and can be set to a view in java code.

Lately android added a xml attribute to views called android:onclick, that can be used to handle clicks directly in the view's activity without need to implement any interface.

Both function the same way, just that one gets set through java code and the other through xml code.

I'm not sure the question is clear. View.OnClickListener is an interface, which defines the onClick(View) method. If you have a class which intends to listen for clicks, you should both implement the interface (if not already extending a class that does), and implement this method too. You have to use both; they're not somehow alternatives.