Dim is a data type(variant) that user can assign to a 
variable like Dim User 
User = "QTP"

Redim can be use to reinitialize the variable. We can give 
a new value to the same variable and can use it again in 
the script.

Dim statement is used to formally declare a variable, where 
as ReDim statement is used to resize a dynamic array that 
has already been formally declared.

Dim Statement

Declares variables and allocates storage space.

Dim varname[([subscripts])][, varname[([subscripts])]] . . .

Variables declared with Dim at the script level are 
available to all procedures within the script. At the 
procedure level, variables are available only within the 
procedure. 

You can also use the Dim statement with empty parentheses 
to declare a dynamic array. After declaring a dynamic 
array, use the ReDim statement within a procedure to define 
the number of dimensions and elements in the array. If you 
try to redeclare a dimension for an array variable whose 
size was explicitly specified in a Dim statement, an error 
occurs.


ReDim Statement
Declares dynamic-array variables, and allocates or 
reallocates storage space at procedure level.

ReDim [Preserve] varname(subscripts) [, varname
(subscripts)] . . .

The ReDim statement is used to size or resize a dynamic 
array that has already been formally declared using a 
Private, Public, or Dim statement with empty parentheses 
(without dimension subscripts). You can use the ReDim 
statement repeatedly to change the number of elements and 
dimensions in an array.

If you use the Preserve keyword, you can resize only the 
last array dimension, and you can't change the number of 
dimensions at all. For example, if your array has only one 
dimension, you can resize that dimension because it is the 
last and only dimension. However, if your array has two or 
more dimensions, you can change the size of only the last 
dimension and still preserve the contents of the array. 

The following example shows how you can increase the size 
of the last dimension of a dynamic array without erasing 
any existing data contained in the array. 

ReDim X(10, 10, 10)
. . .
ReDim Preserve X(10, 10, 15)
Caution   If you make an array smaller than it was 
originally, data in the eliminated elements is lost.