The concept is pretty simple, (like all get ideas) instead of writing down something in a linear form you write whatever coming into your head firsts relating to the core subject. You categorize new subjects as connected balloons to the main subject and continue on till you think you have everything jotted down. It works great for me because now I don't have to try and force my brain to think of a project from start to finish while trying to organize it.
Start by putting your core subject into the center of a piece of paper and then collect your thoughts or data in satellite areas around your core idea. Here are links for some more in-depth mid mapping insight.
wikipedia
The inventor of Mind Mapping
Also if you are a Macintosh user this is free software I used to create the sample mind map.
Hers a funny mind map of how ideas can get messed up by too many people inputing there opionions.