Precondition is nothing but the Expected output where as 
Post condition is also Expected output but it will be 
dynamic i.e it will be many 

    Precondition will be one but post condition can variey 
it may be 1 or it may be many.

Can anybody give an example to justift the above answer?

pre condition is that upon which u write a test case.
ex:-check login with invalid username and pwd.u have to 
write a test case

postcondition is what output you get after giving your 
input.means your expected out put , that can be maney.
ex;-you get aerror message.,you get a new prompt etc.

Above two answers perfectly wrong

Pre Conditon is nothing but 
 1. The data which is used to execute the test case.
 2. The Environment details

as per my knowledge there is no post condition.

u r totally wrong Nag...Please verify ur answer!!

We will take an eg. of  Login Page
Precondition (prerequsite) : S/w should be install on 
system,login page should be available on desktop
postcondition : login successful.

Precondition is nothing but the settings/conditions  
required to execute the test case.
Ex:Precondition for pen is as follows
1.Pen should have sufficient ink
Steps:
1.Write on white paper using same pen
Expected out put:
1.All the letters should be properly visible

Note:Precondition is different from expected output
I can tell post condition is nothing but the expected out 
put.

all the answer above are wrong ..

Precondition is your expected input data and post condition
is your expected values

piush is rite

a postcondition is a condition or predicate that must always
be true just after the execution of some section of code or
after an operation in a formal specification. Postconditions
are sometimes tested using assertions within the code
itself. Often, postconditions are simply included in the
documentation of the affected section of code.

For example: The result of a factorial is always an integer
and greater than or equal to 1. So a program that calculates
the factorial of an input number would have postconditions
that the result after the calculation be an integer and that
it be greater than or equal to 1.