However, parallelization along the trailing edge is vastly more difficult. In general, if the type of processing that you will expect to perform will benefit from parallel processing, make sure that the DBMS you use will allow you to physically structure the disks so that parallel processing is not limited to the physical sequence of the rows. The difference between SMP and MPP is what is shared. In SMP, the processors share a common memory, while in MPP the processors don't share anything. SMP machines are easier to control while MPP machines scale much farther.
Clusters are groups of machines joined through some type of high-speed network like connection.
The relative advantages of SMP versus MPP are best left to another discussion. However, the advantages of clustering are worth discussing. The range of query loads in a data warehouse is truly astounding. Some queries take micro-seconds and some take hours. The data load process itself is usually an infrequent process that is composed of a small number of very large complex processes.