Philosophy of psychology typically refers to a set of issues at the theoretical foundations of modern psychology, much as philosophy of physics deals with questions about fundamental concepts or explanatory strategies in physics. In physics, philosophers have asked about the nature of gravity--is it an actual force or a stand-in for some detailed explanation to be given later?--and about the significance of quantum phenomena for our notions of certainty. In psychology, the questions concern similarly foundational concepts--what is a cognitive module? what psychological phenomena count as knowledge? what is innateness?--and the problems raised from contemporary research such as the question of humans are actually rational creatures or not.
In this way, philosophy of psychology typically concerns itself closely with the work conducted in the cognitive sciences of psychology, neurobiology, artificial intelligence, and such. Philosophy of mind, by contrast, has been a well-established discipline since before psychology was a proper field of study at all and concerning questions about the very nature of mind, the qualities of experience, or the debate between dualism and materialism. These issues arch over the generally more technical concerns of philosophy of psychology, and it may be said that all psychology and philosophy of psychology exist as subdisciplines of the broad projects in philosophy of mind.