A Mindmap is a diagram representing words, ideas, concepts, tasks or other information. Subjects are linked to each other around a central idea. A mindmap is organized in a non-linear way, that makes it easier to memorize than ordinary linear lists. When you create a mindmap you use both the logical and the creative parts of your brain.
The visual technique, using branching and radial maps date back centuries, but was popularized by british psychologist Tony Buzan in the 1970's.
There are many sites about mindmapping, here are a few links
Christoffer Toft Poulsen