Human-Computer Interaction (or Computer-Human Interaction, the two are terms are effectively interchangeable) is the academic discipline that studies how people interact with computers. It builds on and complements parts of two other academic disciplines:
Cognitive Psychology studies the mental processes behind human behavior. That includes such things as perception, learning, accessing information, memory, and problem soliving. Each of these mental processes is a factor in computer use.
Human Factors (or Ergonomics) studies how the design of products affects people. It builds on cognitive psychology and complements this body of knowledge with ergonomics -- the study of human capabilities and limitations vis-a-vis tool use -- and anthropometry -- the study of human body measurements.
Most of the academic work in UI Design is done under the label of HCI, and one of its professional organizations -- ACM SIGCHI -- is probably the most influential forum on user interface design. Many professionals in UI Design, however, have degrees in Cognitive Psychology and Human Factors.
A number of HCI specialists focus on interaction design. This perspective is of particular importance on this site. I like the explanation of what this means by Theodor Holm Nelson (Laurel, 1990, p. 243):
Leaning to program has no more to do with designing interactive software than learning to touch-type has to do with writing poetry. The design of interactivity is scarcely taught in programming school. What we need in software is what people are taught in film school, at least to whatever degree it can be taught. Designing for the little screen on the desktop has the most in common with designing for the Big Screen.
What's the difference between interaction design and user interface design? There's no firm consensus on boundaries in the industry, but consider this clarification from Cooper Software Design's description of their services:
Interaction design answers the question "how should this product work?" It tells us how the elements of the product work together in order to both make its functioning clear and enable the user to undertake her most important tasks easily.
Interface design answers the question "how should this product present itself?" It tells us how the product should look in order maximize readability for the user, and includes the aesthetics of the product.
Interaction design seems to me to have enormous applicability in information design, too.
