Functional defect: If you given a login form having username and password,
and if you give the valid user name and password but it wont accepts means it is
a functional defect.

Usability defect: The usability is how the application is user friendly to
the customer. It depends on the front page of the application. It should be
understandable to the customer.  If the proper buttons or fields are not
accessable by the customer is usability defect.