Clustering technology, allows a network of servers to share resources for better performance and uptime. Clustering are basically of 2 types, as they meet two main goals...

Clustering technology for Load Balancing allows a network of servers to share the load of traffic from clients. By load balancing this traffic across an array of servers, access times improve and reliability increases. Additionally, since many servers are handling the work, one failure will not cause a catastrophic breakdown. This kind of service has tremendous value to companies with extremely high-traffic Web sites.

It works like a high performance cluster in that it's designed to take advantage of the fastest available hardware in the cluster for whatever task is thrown at it. But, it actually does this by load-balancing the various tasks across multiple machines.
Linux servers to provide high availability without the need for expensive hardware. The cluster is totally software-based, with the servers communicating over a high-speed network.
Linux HA Cluster can be configured for fail over, where a backup server sits at the ready to take over for its failed counterpart. It can also work to make the cluster appear as one "virtual server," balancing the load of traffic between the servers in the cluster.
The outside world sees only one address for the virtual server. This helps to insulate the individual machines within the cluster, and allows the cluster to dole out traffic to the least busy servers send traffic to whichever server is up and running.
In fact, when you launch an application, you can do a "ps". It'll appear to be running locally even if it's really running on the idle quad-Xeon server in the corner.

Clustering technology for High Availability servers involves having the servers act as live backups of each other. This is called "High Availability Clustering" (HA Clustering) or "Redundancy Clustering."
By constantly tracking the performance and stability of the other servers, a high availability cluster allows for greatly improved system uptimes. This can be crucial in high traffic e-business sites and for other mission critical applications.
The servers of a high availability cluster don't normally share the processing load that performance cluster servers do. Nor do they share the traffic load as load-balancing clusters do. Instead, they sit at the ready, able to instantly take over for a failed server. Although you won't get increased performance from a high availability cluster, their increased flexibility and reliability make them necessary in today's information-intensive business environment. High availability clustering also allows for easier server maintenance. One machine from a cluster of servers can be taken off line, shut down, and upgraded or worked on without taking the services provided out of commission. When that server is all set, it can be brought up, and the next one in the group can undergo maintenance.

At its core, clustering technology has two basic parts. The first component, made up of a customized operating system (such as the kernel modifications made to Linux), special compilers, and applications, allows programs to take full advantage of clustering.

The second component is the hardware interconnect between machines (nodes) in the server cluster. These interconnects are often highly specialized interfaces. In some cases, the hardware is designed specifically for clustered systems. But in most common Linux cluster implementations, this interconnect is handled by a dedicated fast Ethernet or gigabit Ethernet network.

Assignment of tasks, status updates and program data can be shared between machines across this interface, while a separate network is used to connect the cluster to the outside world. The same network infrastructure can often be used for both of these duties. However, doing this can cause performance to suffer when network use is high.