A question tag is the "mini-question" at the end. A tag question is the whole sentence.
How do we answer a tag question? Often, we just say Yes or No. Sometimes we may repeat the tag and reverse it (..., do they? Yes, they do). Be very careful about answering tag questions. In some languages, an oposite system of answering is used, and non-native English speakers sometimes answer in the wrong way. This can lead to a lot of confusion!
Answer a tag question according to the truth of the situation. Your answer reflects the real facts, not (necessarily) the question.
For example, everyone knows that snow is white. Look at these questions, and the correct answers:
In some languages, people answer a question like "Snow isn't black, is it?" with "Yes" (meaning "Yes, I agree with you"). This is the wrong answer in English!
Here are some more examples, with correct answers:
The moon goes round the earth, doesn't it? Yes, it does.
The earth is bigger than the moon, isn't it? Yes.
The earth is bigger than the sun, isn't it? No, it isn't!
Asian people don't like rice, do they? Yes, they do!
Elephants live in Europe, don't they? No, they don't!
Men don't have babies, do they? No.
The English alphabet doesn't have 40 letters, does it? No, it doesn't.