My son does not have Down syndrome, but he has a chromosome disorder that has a lot of similarities (Pallister-Killian syndrome, chromosome 12). As a PP noted, Down syndrome can affect people differently, from milder mental impairment to severe-profound impairment. Some have physical problems, such as heart complications which require surgery. You can find tons of information on the internet about those things.
I can tell you a little about living life with a child with severe intellectual impairment--you love them the same. No, you love them more. You love them for all the things they can't do and you push them to try new things. You learn about therapies you never even heard of and sometimes they work for your child. When they don't, you move on. Accomodation will become your mantra. You will do whatever it takes.
My Christian is severely mentally handicapped. He is also autistic, hearing & vision impaired, and has seizures. Sounds bad, doesn't it? But he is so lively and fun and friendly. He learnes new things all the time. Just this month he learned how to swing! Christian is loving; he doens't hold a grudge. He isn't sneaky. He doesn't negotiate. He can be naughty, in the way that a 1yr old is naughty. He never tries to hurt someone and he lives for a good hug.
I have two older kids, one with bipolar disorder and ADHD. I must say, raising Christian has been a piece of cake compared to raising DS21.

Best of luck to you! You're in for the ride of your life