I believe, где is where? and куда is like, to where? or something like that I am not sure D:
Где means the place itself without action, and
куда means direction or aim.
Not sure, I'm right, but it seems like you have to manipulate time to male this difference between
куда in English.
Где maps simple time, and
куда maps continuous and perfect:
- Где эта улица? - Where is this street?
- Где было молоко? - Where was the milk?
- Куда ты идёшь? - Where are you going?
- Куда ты положил молоко? - Where have you put the milk.
PS: There can be the other meanings of this words, I was answering only about specific case.
It's deeply rooted in Russian grammar. Location without movement governs the locative, while location with movement uses the accusative.
Где -> no movement, locative Куда -> movement, accusative
Something similar goes, for instance, in German:
wo -> no movement, dative wohin -> movement, accusative
In English movement can be expressed directly with "where to", but often is indicated by other parts of the sentence, or not at all if that's implicitely clear. In Russian you simply must know whether there is movement or not, or else you won't be able to form a proper sentence.
Beside the need to use the proper case, "where" is not the only word that has two variants. For instance, "here" ("from here" / "to here") will also be said differently according to movement.
To show how strong a meaning these adverbs carry, these two sentences are common in Russian:
Где ты? -> where are you? Куда Ты? -> where are you going?
While in English the meaning is carried by the verb, in Russian the adverb is sufficient.