Type identity thoery, or type physicalism is the theory that brain activity and corresponding mental processes are type-identical.
The term type in this version of identity theory is meant to signify a difference between type and token, in which type is a category and token is an instance of a member of that category. For instance, the phrase “bigger and bigger and bigger” contains two types of words (the types being “bigger” and “and”) and five tokens (three tokens of the type “bigger” and two tokens of the type “and”).
So, activity in a specific type of tissue or area of the brain may correspond with pleasure, making pleasure and that brain activity type-identical.
Token identity from type identity in that the relationship between a physical and mental event is more specific to the event. A specific token of a physical occurance in the brain corresponds with a specific token of a mental activity. Token identity theory more broadly states that mental and physical processes generally refer to the same thing, though they need not be neccessarily of a specific type on either side. Token physicalism is, in essence, the same as property dualism which holds that mental properties of the brain (mental tokens) correspond with physical properties of the brain (physical tokens) — both of which are analogous to each other and refer to the same physical object, but which differ ontologically in that mental processes are of a different quality than their physical counterparts.
See: mind-body problem