NoClassDefFoundException:
"Thrown if the Java Virtual Machine or a ClassLoader instance tries to load in the definition of a class (as part of a normal method call or as part of creating a new instance using the new expression) and no definition of the class could be found."

ClassNotFoundException:
"Thrown when an application tries to load in a class through its string name using:

* The forName method in class Class.
* The findSystemClass method in class ClassLoader .
* The loadClass method in class ClassLoader.

but no definition for the class with the specified name could be found."