The Amish are an old German pietistic order. They don't associate with the 'English', as they call the outside world, except to sell baked goods and such. They are monogamous, their marriages are not arranged, and every young Amish has the opportunity to go out into the world for a while to see what it is like. I don't think most of them do this, but they all know that they can. Schools are well-organized to teach what every Amish needs to know about housekeeping, making clothes and even shoes, cooking, managing money, carpentry, animal husbandry and real estate, because their communities are constantly growing.
Unlike the FDS Church members, the Amish live entirely within the law and are respected by the community. I used to live on a farm near some Amish farms near Charles County, MD, and the Amish would trade favors with the local non-Amish farmers quite often, buying fencing and other materials in bulk to get a wholesale price. They would trade pies for apples and cherries, and occasionally taught neighbor's young daughters how to cook.
I have never met any FLDS types, and most of what I know about Amish is hearsay from a son of a landlady I had in MD who had some Amish neighbors.