If you’ve ever tried to set up a Wi-Fi HotSpot, you may have already discovered that you need more than a broadband Internet connection and wireless router or access point. Off-the-shelf routers and APs don’t provide the “captive portal” function needed to either authenticate users or just let them know who to thank for their free connection. Nor do they usually provide other features such as billing support, bandwidth limiting and user isolation. To obtain hotspot-specific features and capabilities, you must use a device commonly referred to as a hotspot gateway.
via How To Build an Open Source Wi-Fi HotSpot with DD-WRT – SmallNetBuilder.