I think they are quite different...
Raita that I have had is runny and has carrots and other ingredients in it that tzatziki does not. Tzatziki is *drained* yogurt, so it is thick and not runny at all, and has garlic (which I think raita lacks), lemon juice, cucumber of course, and sometimes dill or mint.
According to Lebanese people, the Greeks stole many foods including this one from the Lebanese. LOL I say that they simply knew what was tasty and made it in their Greek homeland rather than having to sail for it. In any case, the Lebanese version consists of the same ingredients but the cucumber is in small bite sized cubes rather than grated and drained. I don't know which I prefer, I'm addicted to both lol. My ex used to say "but don't put that green stuff in it please"- meaning mint. Dill went over well though...