They are all potential domiciles for your new Sim family. By moving the pointer over the various houses, you will see the addresses, who lives in each home, the family's net worth and the number of friends they currently have. If you are hovering over an empty house, a pop-up window will list the address and how much it costs to move in. If you hover over an empty lot, the window will display the address and how much it costs. If you buy an empty lot, you'll still have to pay to build a home there.
Different lots cost different amounts, depending on their size and locations.
To play any home, simply hover over the residence and click the mouse. Use this feature to navigate once you've created a number of families to move between all of them. This will allow you to populate current structures. If you put a new family on a vacant lot, you'll have to build a structure to house them there. If you want to move into a home that is currently being occupied by other Sims, you have the choice to Evict them, if you wish.