Usability refers to how easy it is for a visitor to your site to use your site in its intended manner. In other words, are navigation, content, images, and any interactive elements easy to use, functioning the way they were intended, and that your intended target visitor will not need any special training in order to use your site.
The term "user experience" refers to a concept that places the end-user at the focal point of design and development efforts, as opposed to the system, its applications or its aesthetic value alone. It's based on the general concept of user-centered design.
A User Interface (UI) is the face or shopfront through which a user interacts with an application. It describes the way that the user uses input devices such as keyboards and mice, and the way the information is portrayed on screen or on the output device. A Graphical User Interface (GUI) uses visual controls such as menus and buttons to allow the user to accomplish tasks. The user typically uses a mouse. A console or command-line interface requires the user to type commands as text using the keyboard. A web site can be considered as having a User Interface. The term User Interface describes the way in which the user interacts with the Web site.