Usability and experience design has been around as long as complex systems have been engineered
Engineers are good at making things work; not necessarily how to make things work for people
Usability and experience design are a combination of accumulation of experience in craft and critical thinking. They can be considered technical design.
The difference between usability and user experience design can be viewed the same way the different functions of an architect versus an engineer come into play when designing and constructing a building. The architect designs the building and aims at creating a vision that meets the function of the space, but also may illicit emotion or convey a sense of history concerning the space or site on which the structure is built. The engineer, on the other hand, is responsible for making certain that the structure can be built, is efficient, and is in line with building practices that will create a stable structure.
In this scenario, user experience design is the architecture; usability design is the engineering.
Usability design is centered around making the interface to an application usable and function as efficiently for the user as possible. Experience design centers around making the experience resonate with the user and leaving the user with a sense of satisfaction stemming from their interaction with the design.
It is important to note that usability and experience design feed each other in significant ways : a user may not have a satisfying experience if the application is not usable and is therefore a point of frustration. On the other hand, if an application is simply usable it may not create an emotional relationship with the user, and therefore may not leave the user with a satisfying experience.
The best user interfaces are therefore usable and designed toward creating a satisfying user experience.
In essence, both usability and experience designs may be validated through testing. Testing usability, however, tends to be more quantitative and objective in nature. Testing experience is may be more observational and subjective.