How are server computers made more reliable than desktop machines?

Server computers are just slimmed down desktop alot of times. They don't have the fast graphics cards, etc. that a desktop has. They typically have a large amount of system memory, a large harddrive, and a multitasking cpu. The real important thing about most servers which makes them reliable is the fact that many of them operate on what is called RAID. They have multiple harddrives in the system that hold multiple copies of the same data. If one drive fails the other one can pick up where the old one left off and the system isn't compromised. You can do the same thing with your standard desktop, but most people don't want to spend the money on the extra drives required for this.