Psychology is the study of the human mind and as such, is hard to provide concrete evidence. It's virtually impossible to prove that something *is* and instead psychologists usually prove that something *isn't* (and with enough "isn'ts", you can usually end up with an "is").
Biology is the study of plants, animals, humans and other living things - literally how they work, why they work etc.
Biology and psychology work together in the study of mental health problems which cross the hazy line between psychological conditions and physiological conditions. That is, some conditions (such as Tourette's) are assumed to be mental problems, but are in fact caused by incorrect electrical signals in the brain, which is a biological condition. Depression, however, is not necessarily caused by a biological complaint (although it can be in the case of faulty hormone levels). Therefore in the study of mental health, a working knowledge of biology is extremely useful.
