They are a special type of building that can be found in any category (residential, business, and decoration). They are special buildings that have really strong stats.
Landmarks are pre-placed on the map, and are unlocked once the property where they are built is opened for business. At that point, they become a building you can freely move around depending on your objectives.
Once unlocked, landmarks start appearing in random card packs, and can then be upgraded like any other building, if you find upgrade tokens for them.