The Webhosting Models
There are many elements that make up webhosting - web servers, database servers, email, and file servers just to name a few. One server with multiple sites hosted on it is known as a Shared Server. These sites have certain things delegated to the customers account such as RAM, CPU, and Storage. Some customers want to host website themselves. A recommended plan is set up for the customer determining the space the customer needs on the server and the type of services the customer will be providing for their site. Dedicated Servers are the next step up.
Dedicated Hosting is become more versatile, the Server providing the service can be a Virtual Dedicated or Dedicated server. A Virtual Dedicated Server or Virtual Private Server (VPS) has its own resources that cannot be allocated by another operation. A customer would have access to the VPS to control the server themselves. This gives customers the ability to maintain and patch the server as if it was in the basement of their house. Dedicated servers are a hosting service that is provided by most Web Hosting companies. At this time the customer gets full control over a server as long as they stay within the terms of the SLA. This could mean the customer has the root access for administration on Linux or administrator access to the Microsoft Windows Server of Choice.
Some services are managed. This is where the customer gets to have their own Web server. However they are not allowed full control and have to manage data through FTP. When a customer starts to modify a server and does not understand the full complexity of a Server than problems in the configurations could occur.
To have multiple servers with the same information a clustering is needed for redundancies. These are for servers that availability is high.