Usability means that the user can quickly and easily figure out what to do on her own with no training.

Training session

In a training session, the software trainer starts off by explaining any underlying concepts and how to use these.

An example of a concept is layers in Photoshop. Another concept is a template in Powerpoint. Or a BCC in Outlook. You need to understand the concepts to fully use the software - or often to use the system at all.

Sometimes concepts can be very abstract with no easy metaphor or analogy - eg pointers to pointers.

The software trainer introduces each concept with a simple example - then she gets the class to all do a hands-on session. The exercises are designed so that it's easy for everyone to see what to do and what the results should be.

The trainer is there to help when you get stuck and also the results do not matter - it's only an exercise.

Usability is the opposite

1. The user is on his own.

2. The concept better be simple or already understood. Think shopping cart, photo album, appointment diary, ...

3. It's not just an exercise - it really does matter to the user. He really does want to buy a shirt / contact a friend / tax his car / ...

4. It really does matter to the site owner that the visitor can complete his task.