it depends....
you can just mirror the systems, and when one goes down, the other one is brought online -- we do that for our big systems, its about a 10 min crossover to change the IP address of the backup. Once the original is fixed they swap roles and the old original is now the backup copy... etc.
there are other ways to do it. That is a physical computer solution, 2 distinct servers.
Virtualization can be used to do the same thing; it just shifts the load to the surviving server is either of the 2 goes down. When both are up, it load balances between the 2.