User defined functions are encapsulation of segments of

code which is used several times in the script.They can be

either within the QTP script or can be maintained in

external .VBS or .TXT files.

User defined function is nothing but we write our own
functions to perform set of action which contains a login to
execute some steps on the application without making use of
inbuilt qtp functions or which contain full logical code by us. 

For. Eg:
If you need to do some arithmetical operation and just
return the value to some object (textbox) or some thing
else. Then write a funcion on ur own and pass the value to
do the operation and that's it.

Similary if u want to do some event handling like check,
click, select object, do some decission making etc. based on
return value u can make use of functions effectively.

Hope ur got the answer.

Thanks & Regards

Arun*
BOSTON, USA

user defined function is nothing but ,u can define ur own 
fuction for particular functionality.
Normaly we will create user defined functions for the 
reusable functionality(which is available in ur 
application)

Who mark that arun answer as wrong,

he given nice answer, yes we can make qtp to perform an
action based on return value of function.

User Defined Function: It is a block of statements which is
used to perform particular task and get a return value,
which is reusable code put in the function and call at ur
desired place, and get the return value of your input
parameters.

make the qtp to do action based on that return value. use
that value at your desired area.

thanks
Baba Fakruddin