User Experience might sound the same as UI, but it is completely different. It is mostly a back end process which determines how the application will interact with the user. UX should be smooth navigation between all the functions of an application. If the application is efficient and runs smoothly, it gives the user a much more efficient experience as the time of a user is the most valuable thing. User experience is determined by its ability to interact with the user interface elements created by UI designers.
UI designers also assist the UX designers in creating an interactive platform, which is why they are confused for being same. But the UI designers decide how the UI will look, whereas the UX designers are the ultimate in charge of how the UI will operate.