Cases in the Russian language, make or break what you are trying to say. In this situation, you're looking at three different cases: Genetive, Accusative, and Preposition (i'll use the same words that Duck has)
-- куда (where), туда (there), сюда (here) -- Are associated with the Accusative case, which indicates a sense of motion. «Куда ты идёшь?» -> Where are you going?
-- где (Where), там (there), тут (here) -- Are associated with the Preposition case, which indicates location (in this situation) «Там парк.» -> There is the park.