I have several servers for various clients with multiple server set ups. Since I have no idea what you're asking, I'll just throw an example out there.
One client of mine, who has a social networking website, has 6 servers.
- 2 Load Balancers
- 2 Web Servers (In the cloud)
- 2 Core Database Servers
This system is set up for complete redundancy. The load balancers transfer traffic to the proper web servers in the cloud. We chose to put the web servers in the cloud so we can expand within minutes. If you know your traffic you can potentially use physical servers, it's up to you. The webservers then in turn query the database servers and immediately replicate across both of them for writes and redudnancy (we're talking milliseconds). Typically, one would do a master slave set up, but for our sake and a few other clients we replicate it instantly for a master master. In this case, we have 2 core database servers sharing the load as well as they are exact duplicates of one another in case something does happen to the data.
This scenario is completely redundant so if a LB, web server, or core db server goes down, we're fully covered for optimal uptime.
I've attached a Screenshot for visual. It may not be EXACT for the super nerds, but it was used for visualization purposes.
Our more serious set ups also have dual power supplies, multiple electrical circuits in our cabinet, and dual NIC cards in case of extrememe emergencies.
We also make backups every night, week, and month and send them offsite to another facility in Georgia. For our more serious clients we also make backups every 2 hours.
Hope this goes into some real world example for you.