Info Needed : What is cloud computing

I want to learn what is cloud computing. I have no Idea about this.
Actually I have Application, Database etc which we want to move to cloud environment. Please elaborate on the same.
Actually more specifically I want to know-
- What is key factor.
- If it is possible to make reversible.
- I want to know how accurate cloud computing its. I mean everything it’s positive sides and negative sides.
Waiting for your help.
Thanks in Advance.
