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.

Functional Defect:- If the functionality of the application is not working as per the Software requirement specification(SRS), it is functionality defect.

Example: IF You enter any password and that text box limit is 1-10 Alphanumeric, and u entered more than 10 orless than 1 alpha numeric then this is a Functional defect.

Usability Defect:-  in this defect application is not user friendly, not easy to use, no proper validations msg.

Example: If You entered password with exceed the limit then proper validation msg should be come, like "please eneterd pwd between 1-10 Alphanumeric.