Both should be used at the time of Input domain testing. 
Input domain testing can be classified into 2 types 
1. Length testing  2. Value testing

Length testing gives how many values to be entered for this 
BVA is to be used that is values that are on the boundary 
of equivalence classes.

Value testing gives what are the values to be entered for 
this ECP is to be used that is it gives valid inputs and 
valid outputs.

Ex:lets take an User ID field--> Enter 1 to 10 characters 
comes under BVA,,, Enter only alphanumerics without any 
other characters comes under ECP,,,

i think u have clarified with this plz let me know if u 
clarified, if no sorryyyyyyyyyyyyy

a)Boundary Value Analysis and Equivalence Classes are two 
techniques in Black Box Testing.

b)while writing Test Cases we can use these two techniques.

Boundary value Analysis:
                       It is nothing but a selection 
technique in which testdata are chosen to lie along certain 
boundaries.
                        That is imagine you have to write 
test cases for a login field. which will accept 1-10 
characters.So if u use boundary value analysis u have to 
check with the boundaries that is test with 1 character and 
10 characters.then check with 2 characters and 9 characters 
and check without any character and with 11 characters.

                        So that means when u are using the 
boundary value analysis that time u have to check withe 
lower boundary,lower bolundary+1,Lower boundary-1,Upper 
boundary,Upper Boundary-1,Upper Boundary+1.(ie 
0,1,2,9,10,11)

Now Equivalence Partitioning:
                         Equivalence partitioning again is 
also a technique.But here we are using with class not with 
a value.
                         if you have to test a field which 
will accept 1-10 numbers,then test with positive class (1-
10_,negative class (<1) and (>10).so checking with any 
value from the positive equivalence class 1-10 will give 
the correct result and checking the result with negative 
class will be same for all the values in the negative class.

Hope now you are clear...
Thanks...

hi friend  Glamorous thanks ur reply
u will introduce me two new terminologies. can i consider
both for every input field what we are going to test

hi friend Biswal thanks ur reply 
some how i clear........... so we have to categories
equivalence class partition data as  
positive class data and negative class data........... 
and apply it for testing

Keep it simple
BVA: test cases on boundaries. 
how? min--max is the range

min,min+1,min-1 and max,max+1,max-1 

apply above rule for the min max boundary 

Now ECP: divide the input range in the class
one valid and two invalid (for any type of range)
true and false ( for boolean)
so, when u divide ur input into valid and invalid class
there also falls boundaries. Now, apply ur BVA techniques to
minimise the test cases. 

suppose input domain is 1-20 
valid class=1-20
invalid class less than 1
invalid class more than 20

Hope the answer is clear to you

byeeeeeee
kishore

I am testing a template and i set rule for this that is it
should be between 2.9 and 4.8 . The value from cache was 4.8
. when i run the application it failed that
template.According to BVA IS IT CORRECT OR WRONG ?