They are the elements where users can give input to or receive output from.
UI elements can be defined as all the elements that are available for the users to give input to like a touchscreen, a button inside the touchscreen, a hardware push-button or a microphone and the ones that make it possible for the users to receive output from a system like a vibration engine, a display or a speaker.
UI Element modalities
Every system wether it is a product, software or service, has UI elements of some sort and the modalities are related to the different senses of our human body. UI Elements generally can receive haptic (e.g. a button or display), acoustic (e.g. a microphone) and visual (e.g. a gesture) input. The most common output elements trigger the human haptic (e.g. via vibration), acoustic (e.g. a notification sound) and visual (e.g. a display or led) sense.
The job of a UI Designer
A UI Designer is responsible for choosing the correct input and output elements from an interaction and technological perspective. Therefore these designers must have a big knowledge about technology and visual design. A software is an easy and very flexible solution but it might not be the best solution and therefore UI designers must work together with UX Designers as they will evaluate user interfaces to make sure they fit the users' expectations.