The word "cluster", in today's marketplace, can mean many different things.   Here you will find sections on "Load Balanced" and "High Availability" Clusters.

Digital had the technology when they brought out the VAX.  You had many machines all working together as one.  You could add and take away machines on-the-fly, and from a user's perspective, nothing happened.  That was a load balanced - high availablity - cluster configuration.  Today - 20 years later, we are just getting to the point of being able to offer that kind of service to users (whether on a web server or on a LAN). 

The easiest to implement are high availability products as there are several vendors offering software to enable one server to take over for another in the event of a failure.  There are also a few open-source projects on the go that work but are hard to implement - but since they are free, you pay only for the time it takes you to learn.

Load balancing is another story altogether.  There are vendors offering hardware and software to enable load balanced web servers but when it comes to other servers (database servers, file servers,  etc.) the choices are fewer and often are vendor specific (read EXPENSIVE).  Again, the open-source community has a few projects on the go which is changing this landscape as well.

Fault Tolerance is a big part of clustering but is a large topic on its own so I will talk about it here. 

Stay tuned - more to come.

Leon