Keto diet questions

Here is what I know about Keto diet from friends and co-workers. These are just general information of what I know. Keto diet is when you eat less or zero carbs, you would consumed more protein, more fat as part of your diet. Here are some of the questions that I have.. if you are doing keto diet or have any informations please share so we can learnt about it together.

1. How healthy is the keto diet?
2. How safe is it to follow?
3. How many carbs can you eat a day or any at all?
4. Is it safe in the long run?