Three house locations, in the three holds that didn't previously have housing. Also adds housecarls when you become Thane of those holds.
Fixed locations. Each house has the same set up available, though you don't have to build them the same way. They work like this: you first build a single room house. From there you can expand this with a main hall and turn the original house into an entrance way. After that you have three sets of three wings, each with their unique and mutually exclusive options. So one wing has additional bedrooms, an enchanting tower, or a greenhouse, another a storage room, alchemy tower, or trophy room, and the last a kitchen, library, or armory. By default a lot of the customization amounts to what you don't build; your house becomes very cluttered if you build every barrel, crate, and decoration available.
You can adopt two children if you have the right furniture for them to live with you. They can bring home pets.
Marriage is in the default game but there's a handful of unique dialogues depending on who you married and where you're living.
It's not a huge change to the base game. There's a few new items, mostly crafting or children related. The adoption system works with the existing houses if you have Hearthfire, so you're not required to use the craftables houses to take advantage of that. I think it's best to have the DLC early, since you can choose your house sooner rather than move all your stuff into one later. If you're playing on the consoles they're decent enough houses. If you're on PC you have a lot more options, but I've personally been preferring Hearthfire housing combined with some mods to customize them further.