Buildings are special decorations that require time and some materials to construct or upgrade. Materials can be collected from Star Boxes and are sometimes awarded as bonuses for playing Hidden Object Scenes.
Each building that you construct or upgrade will require materials and some time to complete, but once it's been completed, it will generate Coins every so often. You can use these Coins to decorate even more by tapping on a building when the Coin icon appears above it.
You will find buildings in a separate tab in the Shop, and each building has a Star requirement that must be met in order to unlock it.