Keyword 	Description
Empty 	

Used to indicate an uninitialized variable value. A variable value is uninitialized when it is first created and no value is assigned to it, or when a variable value is explicitly set to empty.

Example:
Dim x   'the variable x is uninitialized!
x="ff"   'the variable x is NOT uninitialized anymore
x=Empty   'the variable x is uninitialized!

Note: This is not the same as Null!!
IsEmpty 	

Used to test if a variable is uninitialized.

Example: If (IsEmpty(x)) 'is x uninitialized?
Nothing 	Used to indicate an uninitialized object value, or to disassociate an object variable from an object to release system resources.

Example: Set myObject=Nothing
Is Nothing 	Used to test if a value is an initialized object.

Example: If (myObject Is Nothing) 'is it unset?

Note: If you compare a value to Nothing, you will not get the right result! Example: If (myObject = Nothing) 'always false!
Null 	Used to indicate that a variable contains no valid data.

One way to think of Null is that someone has explicitly set the value to "invalid", unlike Empty where the value is "not set".

Note: This is not the same as Empty or Nothing!!

Example: x=Null 'x contains no valid data
IsNull 	Used to test if a value contains invalid data.

Example: if (IsNull(x)) 'is x invalid?
True 	Used to indicate a Boolean condition that is correct (True has a value of -1)
False 	Used to indicate a Boolean condition that is not correct (False has a value of 0)