On the top of the QTP script if u write Option Explicit 
that means u r going to define every variable u r going to 
define every variable u r using in the script as DIM.
  for an example if u want to define an array ......
  Option Explicit
 Dim A(10)
  

When you use the Option Explicit statement, you must 
explicitly declare all variables using the Dim, Private, 
Public, or ReDim statements. If you attempt to use an 
undeclared variable name, an error occurs.

VBScript ingeneral doesn't need variable declaration. For 
example without using "Dim" a variable can be directly used 
for assigning etc. For ex: temp=1

However, such practise can always be error prone. Ex: if i 
want to use the same variable in someother statement and 
miss spell it,say 

tempe=temp+1

VBScript still considers "tempe" as valid. This case if we 
use OPTION EXPLICIT On top of our application and proceed 
then application will not proceed unless until u declare 
each and every variable. Hence u make sure that ur variable 
is serving ur purpose.

in this example u recognize that "tempe" is not desired. 
Hope this clarifies.

Hi: 
Pallavi, Thank you very much for the elobarate answer.
I got the thing...

Can you also tell me the exact difference between
GUI and UI.

Thank you 
Siva..........

Use Option Explicit to avoid incorrectly typing the name of
an existing variable or to avoid confusion in code where the
scope of  the variable is not clear.

when you use Option Explicit statement,you must explicitly
declare all variables using the Dim,Private,Public or ReDim
statements.If you attempt to use an undeclared variable
name,an error occurs.

You can declare variables with the Dim, Public or the 
Private statement. Like this:

Dim myname
myname = "ritesh"

However, this method is not a good practice, because you 
can misspell the variable name later in your script, and 
that can cause strange results when your script is running.

If you misspell for example the "myname" variable 
to "mynime", the script will automatically create a new 
variable called "mynime". To prevent your script from doing 
this, you can use the 'Option Explicit' statement. This 
statement forces you to declare all your variables with the 
dim, public or private statement.

Put the Option Explicit statement on the top of your 
script. Like this:

Option Explicit
Dim myname
myname = "ritesh"