Usability vs Usefulness

Usefulness measures how the product matches user needs, how relevant the product is to the user. Usability is a measure of how easy the product is to use.

My mobile phone is useful because it has an alarm clock. It's usable because it's easy to navigate to the alarm clock (by accessing main menu).

My mobile is a nokia 3210, I find that nokia phones are the most usable on the market as they have a very simple and easy navigation system and user interface.. and this system is pretty much standard throughout their range of phones, so that if you know how to use one type of nokia phone, you'll generally be able to use any other type.

My mobile lacks some useful features such as a camera or video. But the basic features it does have are very easy to use - i.e. I would give it a high usability rating, but maybe a lower usefulness rating because it lacks some of the features I would like to have, such as a camera.