You're asking about two very different things. Linux is an operating system, which is what the computer (or server) runs on. Windows is another operating system, and there are still others, but Linux is the big player in servers. The Cloud is an organizational architecture for server services: rather than having a specific server (or share of a server), you get assigned server(s) as needed as your computing load varies. If your load is very uneven, you may end up paying less as you don't need to pay full-time for servers you don't need all the time, but what you do pay for is likely to be somewhat more expensive.