Wireless roaming can be very handy if your network is arranged in a way that you can support it. In order for roaming to be possible, your APs all need to be from the same manufacturer, they all need to reside on the same physical wired subnet (i.e., on the same IP network with no intervening routers), and all need to have the same Network name (ESSID).
In the AirPort, roaming is automatically enabled if all of these are true. Make sure that all of your AirPorts have the exact same Network name under Wireless LAN Settings. If for some reason you want to disable roaming, just give each AirPort a different ESSID.