Packages are a namespacing system that also corresponds to the organisation of classes in the filesystem hierarchy.
An interface is a type that requires certain methods be defined. It is referred to either in the heading of a class (as in class C implements interface X) and anywhere a type reference is required, excepting that an interface is not a "concrete" class and cannot be instantiated directly (with new).
package foo.bar.thing; // where in the package namespace the name Baz will be defined
import zik.zot.OtherThing; // references a class or interface in another package
public class Baz implements SomeInterface {
private List<OtherThing> things; // requires elements implement this interface
}
