I just came across a nice visualisation of Dan Saffer's »Disciplines of User Experience« which I accidently found on the German Wikipedia article on »Interaction Design« made by the Munich-based studio »envis precisely«.

Saffer's original is this pretty and more puristic one:

In fact, the altered and more detailed version by envis precisely helped me quite good to get what Interaction Design comprises.

PS: »envis precisely« sounds like »Elvis Presley« to me. ;)