What are the three things that an interface may contain? code example

Example: where do you use interface in framework

Interfaces specify what a class must do. 
It is the blueprint of the class.
It is used to achieve total abstraction. 
We are using implements keyword for interface.

Basic statement we all know in Selenium is
WebDriver driver = new FirefoxDriver();
WebDriver itself is an Interface.
So we are initializing Firefox browser
using Selenium WebDriver.
It means we are creating a reference variable
of the interface and creating an Object.
So WebDriver is an Interface and
FirefoxDriver is a class.