If you look at a WiFi network's flow of information, devices are arranged in a star topology. Every device has a point-to-point connection with your router and information flows through the router for every interaction. If your router is poorly placed in your home or you are a great distance from the router, this single point of failure can wreak havoc.
Z-Wave networks are much more of a mesh in topology. Multiple devices throughout your home are linked to each other with multiple redundant wireless connections. There is still a central controller for routing messages and not all devices are created equal with some being blessed to help route traffic around.
The only dual-mesh network, Insteon uses both powerline and wireless signals to knit together every device in your home. While often desired for sophistication, there is no need for a central controller or router as all Insteon devices can communicate with one another natively and help enhance the overall network.