From what I understand, "是不是" carries the same meaning of "吗". So like the other user said, it is used when asking a question. eg. 你是不是澳大利亚人? is the same as 你是澳大利亚人吗？ However "不是" means essentially means not. So, 我不是中国人. (If I'm wrong, everyone feel free to correct me)

First of all, you should know what does 是 , 不是 and 是不是 mean. In the Q & A sentence, 是 means affirmative answer. 不是 means negative answer. 是不是 is for question. For example: Q: 你是不是学生？ Are you a student? A1: 我是学生. I am a student. (affirmative answer) A2: 我不是学生. I am not a student. (negative) 我是老师. I am a teacher. 有没有 is used in the same way.

They are not students=他们不是学生

They are not students =他们不是学生 Are they students?=他们是不是学生,and This sentence is a interrogative sentence

When a sentence end by "吗”， it is a question sentence. Therefore, "Are you a Chinese?" = 你是中国人吗？ = 你是不是中国人？ “Are you not a Chinese?" = 你不是中国人吗？--> It is usually used when ask a question in reply or a rhetorical question. Do you notice? When a question sentence use "是不是”, there is unnecessary ending by ”吗“. Because "是不是” is already a question sentence.