Specifically people with Downs Syndrome have three copies of chromosome 21. They're not ill and they don't have a disease. There are certain illnesses that they can be more susceptible to and people with Downs Syndrome have learning difficulties, but they are all unique individuals. They don't have the same illnesses or the same level of learning difficulties. Just like everyone else in the world there's no one size fits all. The Downs Syndrome Association has a great FAQ here and the Shifting Perspectives photograph exhibitions which began with this one are fantastic.
My younger sister has Downs Syndrome. She went to the same Primary School as me, a High School for special needs, and then the local college. Her social skills are fantastic but she can't read, write, or work out money. She can swim and earned badges, has done archery and beat our dad at it, loves learning and has lots of certificates. She has set routines and will not go any faster, even if she's running late in the mornings, and is excellent at ducking household chores. She's a huge fan of Star Trek, Glee, Friends, Strictly Come Dancing, Avengers...her DVD collection rivals my book collection.
Things are both different and not different. I graduated from university and my sister received a City & Guilds certificate. We both got graduation cards. I go out at the weekends with my friends and boyfriend and my sister goes out to a special needs disco, bowling, the cinema, and parties with hers but always with us or a carer. She likes to ask me for boyfriend and sisterly advice. Would things be different if she didn't have Downs Syndrome? Sure, things would be different, but my sister wouldn't be. She'd still be as great a sister and as much a pain in my arse a sister as sisters are. And still educating me in Star Trek.
This year for World Downs Syndrome day there's a t-shirt that amused us greatly and a video below that somewhat problematic in that everyone with Downs Syndrome actually has different abilities, but overall it's solid:
And in the spirit of awareness, I am always open to questions and curosity :) (Although just to note I don't speak for or on the behalf of anyone else, just as what it's like to be a sibling of someone with a disability.)