The game progresses by purchasing abandoned buildings and turning them into Housing (which allow you to take in additional Survivors), or Businesses (which allow you to earn Coins by selling Products to buy more abandoned buildings). As you purchase more buildings, your map expands to the right, and more buildings are unlocked.
Every fifth building you buy gives you a Supply Depot, where you fend off hordes of angry Zombies to capture Supplies. You'll also periodically find a survivor trapped in an outhouse yelling "Help!". Purchasing the building in front of them will free them from their capture and they'll reward you with bonuses such as large amounts of various Currency or sometimes upgrades to your wall and front gate (these appear to be cosmetic only and not actually have an effect on the game).
What Stuff Means