Wohin is used to indicate movement/direction toward something - "to where" whereas wo is simply "where" with no movement involved.
So: "Wohin faehrt dieser Bus?" - Where is this bus going (to)?
BUT "Wo ist dieser Bus?" - Where is this bus? [at this exact moment, so no movement]
You'll also find the -hin ending attached to da "there."
Wohin gehen wir? - (to) where are we going?/ Where are we going to?
Dahin. - (to) there.
"-her" works the same way and indicates movement/direction from something.
Woher kommst du? - Where do you come from?
Daher. - (From) there.
When I took high school German we called these wo-/da- compounds :)
I've always kept wohin and woher straight by remembering that "wo" is "where", so that:
wohin ==> where going to (the "-hin" making me think of "hinterlands", somewhere off yonder)
woher ==> where coming here (the "-her" part reminds me of "here")
I also have this habit of making my hands move as if they are sweeping something away and then sweeping back as I think "wohin . . . woher".
I hope you used the "Report a problem" button (just to the left of the "Discuss sentence" button) to tell die Eule that they need to add "To where does this bus go?" as a correct answer.
You're both correct, but the Report is about the only way these things get changed.