Downs Syndrome is a condition where the subject has one extra chromosome than normal; it's named after a guy called Down and used to be known as Down's Syndrome but became more popularly named Downs without the apostrophe in America and the name has become popular in the UK too. Downs people usually have thinner hair, Chinese-looking eyes (they used to be called Mongoloid because of this); they also tend to have fine skin, learning difficulties and it used to be that they had a shorter life-span being more suseptable to some other disorders as they grew older, but more recently Downs people can expect to live long lives... there's lots of other details like the fact that some have a defect in the bone that connects the skull to the spine, where it is a little shorter than usual which means they could die from tilting their head back too sharply... that's about all I know off the top of my head without Googling for the rest.
It is more common when older women have children. It has to do with the age of a woman's eggs. I'm not sure but I think it's because an older woman’s eggs have been exposed to more x-rays, drugs and infections than a younger woman’s eggs.