Definitions and Descriptions of Intrusive Thoughts:
"What exactly are obsessions and compulsions?" from the International OCD Foundation
"Common Obsessions in OCD" from the International OCD Foundation (Note: the terms Intrusive Thought and Obsession can sometimes be used interchangeably, but not always, so watch out.)
"Intrusive Images in Psychological Disorders" from American Psychological Association
"Thinking Bad Thoughts" from Deborah Osgood-Hynes, Psy.D. Director of Psychological Services and Training MGH/McLean OCD Institute
Note: If you know of any good resources that describe intrusive thoughts, please feel free to contact me via comment or wherever and I'll check it out. Thanks!