Down's syndrome is a physical condition caused by an abnormality I believe in the 28th chromosome but don't hold me to that number.
Down's syndrome causes some physical appearance changes typically in the shape of the face and eyes. Historically speaking, it was often called Mongoloid-ism.
Typically and historically, these wonderful people were thought to be mentally retarded, now labeled functionally or developmentally disabled. This is not always the case. IQ is not always effected, though for many they were stunted in their cognitive development by being taught as if they were.
I have worked with this population for over twenty years both in class rooms, day programs and group home settings. They are truly God's children. You will never meet sweeter more caring individuals, full of a natural joy. Of all my students and clients, these were always the ones that grabbed my heart.
I doubt I've answered you question very well and for that I apologize but basically it comes down to this... Down's Syndrome is a label. The people it's assigned to are just that, people just like everyone else but often a lot more fun to be around than those that are so called normal.