A Function procedure is a series of VBScript statements 
enclosed by the Function and End Function statements. A 
Function procedure is similar to a Sub procedure, but can 
also return a value. A Function procedure can take arguments
a Function procedure has no arguments, its Function 
statement must include an empty set of parentheses.

Function Celsius(fDegrees)
   Celsius = (fDegrees - 32) * 5 / 9
End Function

A Sub procedure is a series of VBScript statements 
(enclosed by Sub and End Sub statements) that perform 
actions but don't return a value. A Sub procedure can take 
arguments (constants, variables, or expressions that are 
passed by a calling procedure). If a Sub procedure has no 
arguments, its Sub statement must include an empty set of 
parentheses ().

Sub ConvertTemp()
   temp = InputBox("Please enter the temperature in degrees 
F.", 1)
   MsgBox "The temperature is " & Celsius(temp) & " degrees 
C."
End Sub

The sample application (Flight) that u r using is also one 
of the Client Server applications. 
So, Don't worry : u can say all the application which are 
developed in VB etc.. r comes under Client/server 
applications.

QTP is a functional testing tool.It will be useful in 
execution time.to decrease the time &to increase the 
reliability,reusability in test execution,either it is 
client/server or web based app testing is same with using 
QTP.

PLEASE PROVIDE THE MORE ANSWERS FOR THIS QUESTION.