check out the url  
http://www.allinterview.com/showanswers/118

When We have an requirement which has a large class of data 
split the large class into its subsets 
For Ex: 
Sal 10000-45000
 Here this is the large class of data
equivalence partioning -->take inputs from the below sub 
classes such as
less than 10000 (invalid)
between 10000 and 45000 (valid)
greater than 45000 (invalid)
Instead of choosing values from the large set of 
information split the inputs into both  valid & negative 
inputs that becomes a subset. this technique is equivalence 
partioning

Equivalence partioning is a method of writing test case in 
the black box technique.Usually two invalid inputs and one 
valid input. e.g. Writing test case for a range Rs.1000 to 
RS. 2000.

Valid Test case : 1500 (In between the range)
Invalid test cases : 800 and 2400 (one value- Less than 
lower limit and higher than upper limit)