This is an object that wants Java to automatically serialize all its variables (except the ones declared as transient.) It implements the public interface java.io.Serializable, but there are no actual methods to implement -- this is simply to identify the class to Java as one it should serialize. (See externalizable.)

The ability to convert from object semantics to a wire format.

"When multiple invocations of a method must be invoked one by one in sequence, this is known as serialization."