Someone said :
What's the difference?
I was also confused until the psychiatrist explained it in the following way. (Bear in mind I am NOT an expert, ok.)
Mental illnesses... someone is sick mentally. For the most part, treatment is available. (Not always)
Mental disorders... either
1. It is due to an illness or chemical imbalance or other underlying external cause. Treat the illness or imbalance and the disorder can be treated or managed.
2. It is organic. There is no chemical imbalance or illness. Some people are just born with a personality disorder that is organic in nature. They are usually very dangerous people, and once diagnosed (In the UK) are immediately institutionalised.
Not to treat them, but for the safety of other people.
It's like saying someone with a physical disability is physically sick. They are NOT sick because of the disability. They may be physically well.
EDIT: Thanks for the quote from Wiki. It merely backs up what the psychiatrist told me.