Post-Processual Archaeology is, more than anything else, a critique of processual archaeology.
Archaeology at its best is a study sturdily balanced between anthropology (as the study of human cultures), history (as the study of human historical and prehistoric past), and archaeometry (as the science of decay). Leaning too hard in any one direction pulls the balance out too far (even though it is asking a bit much for one scholar to be all three things). Post-processualists such as Ian Hodder criticized the processualists as getting too involved with the archaeometry of it all and ignoring the stuff of man--the behavior, the gender, the culture of people. Naturally, the processualists think the post-processualists go too far.
There's a surprisingly good article on post-processual archaeology written by anonymous members of the public on the Wikipedia site. A brief bibliography of articles discussing post-processualism was created for this project.