Everything in usability design is about hiding complexity from the user, and making the system approachable as well as enjoyable to interact with. Usability design is truly part art and part science. The only method which is effective when designing a usable interface involves first designing using intuition (driven by persona development and competitive analysis), followed by quantitative and qualitative testing of the design using live test subjects.
Designing for usability is simply impossible to accomplish without validation via testing. At every turn, items that may have been designed with the best intentions for the end user may not perform well with live subjects. In addition, there is always potential that items will fall through the cracks, and some interface aspects may not have even been designed or accounted for . Usability testing is a best attempt at catching the items that have fallen through the cracks, so to speak.
In summary, usability design is :
Users interact with computer interfaces for one of two reasons : because they are told to or because they want to.
In the case of someone being told they must use a computer interface, this is likely job related. Someone must use a computer system as part of their everyday job duties. In this scenario, usability is not as important to enhance user experience, as the user is likely to be forced to engage with a system no matter how usable the interface is.
This is in stark contrast, however, to someone who uses an interface because they want to. This might be a user that interacts with an interface for entertainment purposes, for example. In this case, experience is key; a user will not interact with an interface if it does not deliver something of substance to them, and does not deliver this benefit in a non-frustrating manner.
Usability is paramount because user experience is the most important portion of a system in which users interact.
In a restaurant, patrons do not care if the kitchen equipment is top of the line or of a certain brand, rather they care about the service in the front of the house and the taste of the food. You can have the best equipment or staff in the back of the house, but if the food just doesn't taste good then patrons will leave your restaurant and never some back.
The analogy holds true; the quality of the interactions, the experience, a user has when interacting with a piece of software is the most important aspect for software that is meant for people to use. It does not matter what platforms or tools drive what's outside the user's view, if the interface is not usable then the user is lost and the application is a failure.
The goal of usability design is to mask the complexity of the underlying system and make the afore mentioned system as intuitive and easy to use for the person driving it as possible.
Usable interfaces are intuitive and easy to use; this really means: