hayır - no

Hayır means simply "no" in English. But, sometimes people may say "yok" over hayır. For example;
Sen de gelmek ister bizle? Do you also want to come with us?
Yok/Hayır, teşekkür ederim. No, thank you.
The word "yok" is informal and can sound rude in formal places. And it literally means "there is not".