Interaction design is basically the art of creating user-friendly, intuitable user-interfaces.
Interaction design draws from the related and often overlapping fields of information architecture, human-computer interaction, computer science, software engineering, usability engineering, information science, industrial design, graphic design, web design, and system analysis.
User-interface (UI) design, experience design, and interaction design are arguably synonymous, but each field brings a particular point of view to any design problem. These are not hard and fast rules, but generally speaking, UI designers focus on the graphical user-interface, page layout, navigation, etc. Interaction designers pay special attention to the give-and-take between the system and the user; the actions of the user and the reactions of the system; the process of moving through the system. Experience designers study the user's overall experience; the user's reaction to the system; the user's sense of satisfaction, accomplishment, etc.
Most interaction design work today is done on and for the Web, but interaction design is not limited to medium.