Client mode (also referred to as 'AP Client' mode) allows the router to connect to other access points as a client. In a nutshell, this turns the WLAN portion of your router into the WAN. In this mode, the router will no longer function as an access point (doesn't allow clients), therefore, you will need wires to use the router and to configure it. The router won't even be visible to your wireless configuration software or Wi-Fi sniffer software such as Netstumbler, since it no longer broadcasts beacons. In client mode, the WLAN and the LAN will not be bridged, thus they will be on two different subnets. Port forwarding (from the WLAN to the LAN) will be necessary for FTP servers, VNC servers, etc. that are located behind the client mode router to function properly. For this reason, most users chose to use client bridge mode instead of client mode.
See Also: Client Mode Wireless