Interface

Can someone explain to me what an interface is and how to use an interface.

for example if the interface looked like this....

public interface Data {

public abstract int someMethod();

public abstract int someMethod2();

public abstract String toString();
}


What I thought was that the interface was used to extend another class... but when I attempt something like...

public class useData extends Data () {
}

i get an error like "a superclass must be a class." And I understand what that means... but now I dont know what an interface is.
Thanks