Hi. I just wanted to clarify something
Greek yogurt is regular yogurt that has been drained of the whey, which makes it thicker. So you can make Greek-style yogurt from regular yogurt - put the yogurt in a piece of cheesecloth in a drainer over a bowl and refrigerate for an hour.
Tzatziki is the Greek sauce made with Greek yogurt, chopped cucumbers, garlic, dill and mint. Tahini is a paste made from pureed sesame seeds (like peanut butter), and tahini sauce is a sauce made with tahini, garlic, lemon juice, olive oil and parsley, served with falafel. hth.