Usability is everywhere. Any item you use to achieve something has it, whether it be a stone axe or an Android mobile phone. Some items have better usability than others, but they all have it.
The four main features of usability are
Possibilities is the list of stuff you can get done with the item; constraints list spells out what you definitely will not get done with it. Visibility means how well the parts of the system are visible and thus usable, and order defines whether they are lined out in a good orderly manner (I bet you did not know, by the way, that an ordinary wrecking bar has four parts? Right).
So, in a good object or utensil or system, all four parts are relatively well taken into account, and in a bad one, there’s deficiencies in some of the features.