Nessus

Write a comprehensive study and analysis of Nessus:
What is the use of Nessus tool? Who would use a tool like this?
Disadvantage of the Nessus:
Different Version of Nessus. How they are similar and different. Provide the example
How does the Nessus work in Windows, Mac and Linux? Provide example.
This is what I have done so far edit. I will work on the conclusion part by my self.
Please feel free to add other stuff.