I feel the example given on Functional Testing Good (Only registered user can login this is functionality testing.)

The example given for Usability testing (If user fill wrong user name or password, he get a error message , this is usability testing.) May comes under Error Handle Testing.

Usability testing means say if u have a form which contains 10 fields then " Tab setting " is kept in sequential order like F1,F2...etc

Some specific questions to be addressed in this usability testing:

Will users be able to install the application from a network or floppy with no assistance?
Can a non-PC user successfully launch and use the application?
Can users successfully navigate through the application?
Is the system status clear to users at all times?
Does the tab metaphor facilitate the ability of the user to navigate through the application?
How will users feel about using the on-line help?
Is context-sensitive help a requirement for our users?