What is Usability ?
Usability relates to the how easy things are to use. Generally, usability is measured against five criteria—memorability, efficiency, errors, learnability, and satisfaction (MEELS). http://www.usability.gov/get-involved/blog/2013/01/accessibility-and-usability.html . Although there are other definitions, this isn't a bad one
What is User Experience (UX) ?
The U.S. Department of Health and Human Services has a good definition of UX stating that "User experience (UX) refers to the quality of the user's interaction with and perceptions of a system" http://www.usability.gov/what-and-why/user-experience.html