As you may know, I wrote my honors thesis on Human Computer Interaction back in my university days, and since then have done a lot of UX and UI based work over the years. I thought I would share some insight into some common things that people mistake UX for UI and vice versa. So here is a clarification post for those interested.
Normally, we think of the term user interface (or UI) as it applies to applications. Technically, this term refers to the parts of the software which interact directly with a human. So, it covers things like what options are available to the user at any given time, how those options are presented on the computer screen, as well as the physical interactions (mouse/keyboard, game pad, etc.). For example, with video games, the UI is divided into two parts: the input (that is, how the player gives commands to the game) and output (how the game communicates the results of those actions and other aspects of the game state to the player).