Design
Design is the domain of product managers. It is performed most oten by people who do not do the implementation, however it is strongly informed by the implementation reality. Design is a synthesis from the customer needs and the technical possibilities.
Usability
Obviously we can not attempt to cover all the subtle aspects of creating a usable product as that is a large, subtle, and expansive field requiring much more than this site. however there are aspects of usability that every technologist can understand.