你好 is hello 嗨 is basically just hi basically, 嗨 is very casual 你好 is safer and more formal

Agreed with the previous answers. 你好 hello 嗨 hi

Say 你好 to everybody while say hi or hello to you close friends.

Just add something to @havoy's answer. When you know sb for the first time, you'd better say '你好'. At the same time, you can say '嗨/嘿' to those guys you know each other.

We prefer using 嗨,because it's more international than 你好.And 你好 is more formal than 嗨