You'll get the basic kicks and punches down, but you might commit mistakes with certain moves. This might result in injury or lack of power in the maneuver.
You also need opponents to spar against to properly learn how to defend and launch attacks.
If you just want karate as a form of exercise then I guess a DVD would be fine, but if you want it for self-defense and to properly master the art then you do need to go to a school.