Below is a very useful link for DB2 basics; in addition, IBM Developer Works is a great starting point no matter what IBM technology you may or may not be using.
http://www.ibm.com/developerworks/data/library/techarticle/dm-0503melnyk/
Massively Parallel Processors (MPP)
http://www.ibm.com/developerworks/data/library/techarticle/0203lurie/0203lurie.html