Here are a few important terms related to high availability:
SPOF: Single Point of Failure: Component of a system whose failure impairs the functioning of the whole system.
Failover: Another similar system component automatically takes over the function of a failed component.
Cold Standby: The alternative hardware is on cold standby. The failover must be performed manually, so the failure will be clearly apparent.
Warm Standby: The backup system runs in the background, so the transfer can take place automatically. The data on both systems is automatically synchronized. For the user, the failover is like a very fast automatic service reboot. However, the current transaction may be aborted because it was not possible to synchronize the data prior to failure.
Hot Standby: Both systems permanently run in parallel --- data on both systems is one hundred percent synchronized. Users will not be aware of any failures. This level cannot usually be reached without making a corresponding modification to the client. To run both systems completely synchronously, the connections to the client must be mirrored one hundred percent. This normally requires clients that have connections with two or more servers at the same time and that communicate with all of them. A normal web browser cannot do this.
Load Balancing: The distribution of load within a cluster of computers. Load balancing is used in an LVS scenario (Linux virtual server), for example (see Linux Virtual Server).
STONITH: Shot the other node in the head: Special hardware and software that ensures that a faulty node does not write-access distributed media within a cluster, threatening data consistency in the entire cluster. This involves simply disconnecting the system from the main power supply.