Typically people think of this as a Greek food, but I learned to make it from a friend when I was an exchange student in Germany. My friend, Hossein, was from Tehran, Iran (Persia) and he liked to have tzatziki with every meal (it is a condiment served on the side). As far as I can tell, most of the countries on the eastern side of the Mediterranean Sea, and other middle eastern countries like to eat something similar to this with many of their meals.
Tzatziki is a yogurt salad typically made with cucumbers, onions and garlic. At least, these are what I consider to be the essential ingredients. I have had some in restaurants that omitted the cucumbers. Eh. Not so good, in my opinion.
One of the dinners I usually serve with tzatziki in my home, is grilled salmon with rice, tzatziki and diced tomatoes on the side.
I don't really follow a recipe for this, but here is my approximate amount of ingredients. For yogurt, you can use full, low or non-fat but I don't like the texture of non-fat. I prefer Greek style, it's smooth, thick and creamy not like gelatin.