On many hubs, you will find an uplink port. They usually share a connection with another port - for example, on a 5-port hub you can't use port 5 and the uplink port at the same time. The uplink port is used to connect the hub to another one without using crossover cable - you connect a cat 5 patch cable from the uplink port to a regular port on another hub. The uplink port is usually MDI, unlike the other ports which are MDI-X. For more details, you should see my writeup under crossover cable.