From http://www.w3schools.com (Copyright Refsnes Data)

VBScript Keywords

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)