I'm by no means an expert on the subject, but I know quite a few kids who have it, just through special ed services. I can explain how trisomy 21 occurs, but I'd imagine you can find that by google.
One thing I've found is down syndrome is really stereotyped, and I've found from the kids I've met that while they do have similarities (facial looks, lower IQ, higher rate of some health problems like heart issues, etc.), they are also very variable. They have different abilities, personalities, etc. It's definitely a spectrum of ability. I've heard/read people saying children with DS are always happy... that is not true. But I think they do tend to have a "nice" outlook on life/ have kind intentions (I'm sure not all, but many). I know a boy who has both autism and down syndrome. That's a challenge.
There's also a type of DS called "mosaic down syndrome" which I don't fully understand why, but the people with this usually have higher IQs than with typical DS.
I recommend the documentary "Monica and David". I thought it was really sweet.
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Mum to two awesome kids on the spectrum (16 and 13 years old).