Down syndrome is a genetic condition caused by an individual having 47 chromosomes, instead of the usual 46. It causes both physical and intellectual delays, but it's very important to remember that those affected by down syndrome are more like others than they are different.
That said, the severity of down syndrome varies person to person. However, most people with down syndrome are able to lead healthy lives, and participate in many of the same activities - including sex and relationships - as other people.
I'm including a couple resources that provide some more information about down syndrome.