I just spent a year and a half working at a DD facility. Very rewarding. Unfortunately, there are no places like that to work where I moved to.
Anyway, if most DD facilities are like mine then I should be able to tell you a little about the difference from psych...at least in my mind.
DD is more of a home, like a nursing home but not. It is for people that are not able to take care of themselves because of profound developmental problems such as MR, autism, cerebral palsy, etc.. IMO, you do have to use more "psych" skills in a DD more than a facility like a nursing home.
Psych, on the other hand, has more of your "mental problems". Bipolar, schizophrenia, major depression, addictions, etc. The place that I did psych clinicals for my ADN was very different than a DD. It was a "hospital" and the folks there had classes all day, along with pharmacological management to prepare them to go back into the "real world". Now, some of them have been there forever and a day but some go home eventually.
Maybe that helps?