This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
I trust you have looked in books and the Java Tutorial?
An abstract class is one which you do not mean to instantiate; it can have abstract methods, implemented methods, fields, constructors, etc.
Imagine interfaces as being very very abstract classes. An interface (ideally) only has methods. All the methods are empty, so you only write a list of method headings. You usually miss out "public" and "abstract." An interface might look like thisYou can't compile that because it has "java.lang" in. If you add lots of documentation comments you will reconstruct the Comparable interface, at least before Java 5 when it was Comparable rather than Comparable<T>.
When your class (unless it is an abstract class) implements an interface it also has to implement all its methods, so as to maintain the contract in the original documentation.
You can instantiate an abstract class several ways, but the one you are probably familiar is to write extends. You can implement several interfaces, so using interfaces has several advantages:
You can implement certain functionality from the interfaces.
A class using the objects can check whether a particular method is available by using "instanceof Comparable" or similar. Much quicker than looking for the method.
I am sure other people will have more points to add.