And what does Brain Training do? Well, you hold your Nintendo DS like a book (with left and right screens), and you basically use the touchscreen to undergo a wide variety of simple, cleanly-designed, interesting exercises intended to make you smarter. Or, at least, keep your brain sharp and fresh and delicious. At the end of your "fun", the game eventually calculates and reports your "mental age" — often with painful/comedic effect — and tracks your progress over the weeks and months of self-education. And that's about it.
Of course, no video game can take the place of actually going out and doing something.
See previous posts on the topic of brain training here.