The DSi is a DSL with upgraded specs, two cameras, an SD slot, and downloadable games.
The 3DS is a DSi with upgraded specs, a 3D screen, a thumbstick, integrated rumble, an accelerometer, and who knows what else.
The really important distinction? When Nintendo says there will be 3DS-exclusive games, they really mean it.
"The worst part about these reviews is they are [subjective]--and their scores often depend on how drunk you got the media at a Street Fighter event." — Mona Hamilton, Capcom Senior VP of Marketing
*Image indefinitely borrowed from BrainBoxLtd without his consent.