Equivalence Class Partion defines all the valid and invalid
values of test data for specific scenario.

EX:Scenario for User id which allows only alphanumerics and
special character"_" only

data matrix for user id field:

valid data:A-B,a-b,0-9,_
Invalid data:All special characters other than "_"

Equivalence Class Partition Splits The Given Input Into 2
Classes..
 Class 1: Valid
 Class 2: Invalid

EX:The maximum amount can be withdrawn from ATM machine is 
   100 to 15,000
 

    Invalid          Valid             Invalid

    -----------100--------------15000--------
Note: Total Number of Test Cases is 3 (2 Invalid and 1 Valid)

An equivalence class is a subset of data that is 
representative of a larger class. Equivalence partitioning 
is a technique for testing equivalence classes rather than 
undertaking exhaustive testing of each value of the larger 
class. For example, a program which edits credit limits 
within a given range (1,000 - 1,500) would have three 
equivalence classes:

< 1,000 (invalid)
Between 1,000 and 1,500 (valid)
> 1,500 (invalid)

Example for Equivalence Class Partion 

Squre root 

squre root of all +ve numbers are real(valid)
and squre root of all -ve numbers are imaginay (invalid)

ex:The maximum amount can be withdrawn from ATM machine is 
  15,000
Now accoording to eqivalance partitioning input has to be 
given in the different ranges like
1. 0-5000
2.5000-10000
3.10000-15000

In eqivalance partitioning positive scenarios will be 
tested as above.

In boundary value analysis negative cases also added like
<0
between 0-15000
>15000