DSM-IV-TR is the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision...

What it really is is a big grey book (I have it myself) that lists most possible mental illnesses, such as bipolar, depression, schizophrenia, etc., and the criteria to "meet" one of those. Basically it is used by psychologists/psychiatrists as a kind of textbook or recipe book to help figure out what is happening with a person...