Both tzatziki and raita are very similar - both use yogurt as the base ingredient. However, tzatziki is always made with thick Greek yogurt and cucumber, and raita doesn't always have cucumber - there can be onions, or tomatoes or carrots or other ingredients too. Also, tzatziki is seasoned with herbs such as dill or mint or parsley and raita is seasoned with spice powders such as cumin and black salt and sometimes with cilantro.