Mental illnesses often have sub-categories, such as Bipolar I and II. If your therapist or pdoc doesn't think you fit particularly in 1 sub-category, but knows you at least have bipolar or whatever the condition is, they will put "NOS" at the end of it. I have Dissociative Disorder NOS, which means that I dissociate and have some traits of DID, however not enough to get the exact classification down...

edit: however it does not mean the condition is less severe than if there isn't the NOS after it. It means you just don't fit into the textbook definition, and quite honestly a lot of people don't. There are a lot of gray areas. What's important is that you get the help you specifically need.