A dedicated server is a single computer in a network reserved for serving the needs of the network. For example, some networks require that one computer be set aside to manage communications between all the other computers. A dedicated server could also be a computer that manages printer resources. Note, however, that not all servers are dedicated. In some networks, it is possible for a computer to act as a server and perform other functions as well.
Many dedicated server providers include a service level agreement based on network up-time. Some dedicated server hosting providers offer a 100% up-time guarantee on their network. By securing multiple vendors for connectivity and using redundant hardware, providers are able to guarantee higher up-times; usually between 99-100% up-time if they are a higher quality provider. One aspect of higher quality providers is they are most likely to be multi-homed across multiple quality up-link providers, which in turn, provides significant redundancy in the event one goes down in addition to potentially improved routes to destinations.
The answer to this question largely depends on two factors. Are you new to server technology and not very interested? In this case, Windows is the easiest option to get where you want to go. Do you already have experience with Linux or would you like to familiarize yourself with this operating system? If so, Linux gives you more freedoms to configure your server, and you will be using an operating system that is less vulnerable to attacks from the Internet. Linux is also open source and therefore free.