What is the difference between GUI and UI?
Keyword - Graphical.
User Interface
In the industrial design field of human-machine interaction, the user interface is (a place) where interaction between humans and machines occurs. The goal of interaction between a human and a machine at the user interface is effective operation and control of the machine, and feedback from the machine which aids the operator in making operational decisions. Examples of this broad concept of user interfaces include the interactive aspects of computer operating systems, hand tools, heavy machinery operator controls. and process controls. The design considerations applicable when creating user interfaces are related to or involve such disciplines as ergonomics and psychology.
A user interface is the system by which people (users) interact with a machine. The user interface includes hardware (physical) and software (logical) components. User interfaces exist for various systems, and provide a means of:
* Input, allowing the users to manipulate a system, and/or
* Output, allowing the system to indicate the effects of the users' manipulation.
Graphical User Interface
A graphical user interface (GUI) (sometimes pronounced gooey[1]) is a type of user interface item that allows people to interact with programs in more ways than typing such as computers; hand-held devices such as MP3 players, portable media players or gaming devices; household appliances and office equipment with images rather than text commands. A GUI offers graphical icons, and visual indicators, as opposed to text-based interfaces, typed command labels or text navigation to fully represent the information and actions available to a user. The actions are usually performed through direct manipulation of the graphical elements.
There are lots of other types of User Interfaces.
See http://en.wikipedia.org/wiki/User_interface#Types
Source - Wikipedia
GUI is "graphical user interface" and UI is just "user interface." GUI is a subset of UI. UI can include non-graphical interfaces such as screen readers or command line interfaces which aren't considered GUI.