The Nintendo 3DS has a lot of cool features, one of which is backward compatibility with DS games. But while convenient, this backward compatibility is far from perfect. Because the screens on the 3DS have a higher resolution (400 x 240 on top, 320 x 240 on bottom, versus the DS's 256 x 192 screens) DS games are either upscaled to fit the bottom screen, or shown in tiny windows on both screens if you hold Start and Select while loading the game. In either case, it's not great.
I recorded a quick video to demonstrate a DS game running on a 3DS, which you can find after the break. As a bonus, I'm pretty sure you can hear my cat toward the end of the video.