First of all there are two test design techniques...one is 
black box and another is white box...These testing 
techniques are totally different from each other....the 
white box is for unit and integration and black box is uesd 
for system and user acceptance testing...by combininf these 
total testing of a product or application can be 
achieved...so that's why both are necessary because they 
have significance advantages different to each other.

White box testing is done by the developers only.. in this 
case they go for code testing means internal structure of 
the application.

Black box testing is done by the testing team.. in this we 
do tesiting on the functionality part by applying valid and 
invalid inputs.......

The person who has domain knowledge on which he is working 
can perform both White box and Black box testing..

Hope u understood clearly

any douts u can mail me to ajay.thomala@gmail.com

In white box testing,we do inspection of code(means this 
type of testing is referred to internal structure of 
software or system)
It may be :-

code coverage:-execute each statement atleast once
branch coverage:used for control statements(most 
prefferable) 

Black box testing is used to check the extrenal behaviour 
of the system irrespective to internal structure or code