Usability Testing is nothing but testing an intended user 
of the system can carry out the intnded task efficiently.

usually this testing is done before going to production
by then all the  bugs found  are fixed and closed.

the test scenarios are used to verify the usability.

mainly the participants are atleast:
1 user representetive
1 test host
1 dveloper
1 business representetive.
usually observers are in seperate rooms or on seperate 
machines,and no.of sessions they do is depend on the 
complexity of the application.
usually in about 10 session most of the uncovered issues 
can be found.

Testing for 'user-friendliness'. Clearly this is 
subjective, and will depend on the targeted end-user or 
customer.

Testing the user friendliness of the software, it depends on
the end user and application.


Please correct me if i m wrong.

By
Prabhu Subramanian
CSS India Pvt Ltd.