Functional Defect: Not full fill the requirmnet as we need.
e.g. If the requirment is after successfully login Welcome page should be opened.
1- Enter Correct login detail---- Welcome page should be come.
2-Enter In-Correct login detail---- Welcome page should not be come.
these are functional requirments...and if it is not working then its called Functional defect.
Usability defect: if the Application is not easy to undersatnd then it is called usability defect.
e.g. if you are entered wrong details in login window then validation msg should be come.

