Invented in it’s current form by Educational Consultant Tony Buzan, the concept of Mind Mapping is pretty simple. It’s a technique for visually recording related information for ease of retention.
The original technique was strictly pen and paper based (and many people still prefer that method), but there are also many programs out there to help out.
FreeMind is one of the better ones, especially when you include the "Free" part!
Here’s a quick brainstorming session I did to put together ideas for a space combat game.
Here’s a closer view (though still not full-sized - sizes are selectable from a range). This gives an idea of how things look close-up.
There are a lot of cool "extras", like the ability to add a "Note" to any node - the icon in the grey node above means there’s a note - when you click the node, the note shows up in a window at the bottom of the screen.
Here’s an example of a hand-drawn mind-map - they can be visually appealing if done well, I’ve seen some that were inarguably gorgeous!