Just as the state of computing has come a long way, so has clustering in general. As Linux matures, so does its ability to handle larger clusters using commodity hardware, as well as mission-critical data across HA clusters.
Clustering under Linux is similar to clusters under other operating systems, although it excels in its ability to run under many different commodity hardware configurations. With the right configuration, however, it can provide supercomputer-like performance at a fraction of the cost. The right configuration, of course, changes upon the needs of the organization and the cluster that is implemented.