standard module:

1)place for variable declarations,procedures etc.and we can 
access these procedures wherever needed in the project.

2)Available only for the existing modules where they are 
used.

class module:

1)Unlike standard module,members of class like object's 
properties,events can be accessed only whenever an instance 
of class is created.

2)We can make use of it whenever an object reference of the 
particular class is created for number of applications

Module
Code  in VB is stored in modules.
Thereare three kind of modules, Class modules, Form modules 
and Stendard modules.
In VB there are common codes we want to execute in several 
forms. We do not want to duplicate the code in all forms, 
we create a separate module containing the procedure that 
implements the common code. This separate module should be 
Standard module.

Class Module
Class modules (.CLS extension) are foundation of object 
oriented programming in VB. We can write code in class 
modules to create new objects. These new objects can 
include our own customiaed properties and methods, although 
custom objects can not have their own events.
We can also use the keyword, New to create multiple copies 
of our objects.

class module
    It is a user defined datatype.
    It have datamembers(methods and variables).
    It can be accesed by creating the instance called 
object if the class is not static. If the class is static 
then it can be acessed using classname.datamember when 
datamember is public.
    Protected datamember can be accesed only by the first 
inherited class.
    Private datamember can be accesed only inside the main 
class.