LAN (or Network settings): This is where you can change the local network settings, including the default IP address of the router itself. (Note that if you change the router's default IP address, which is recommended for security reason, you'll then need to use the new address to access the router's web interface.) Here you can also change the range of IP addresses used for local clients, and add clients to the DHCP Reservation list. Once on this list, the clients' IP addresses will remain the same, which is required for some internet applications. Most of the time, you don't need to change anything in this section at all. Just connect the pc's to the router. Then set a home group and check here - Wireless network without internet
Tools (or Administration) section
Admin password (or Password): Change the router's password. This is the password required when you log in the router's web interface.
System: Where you can back up the current settings of the router to a file, or restore settings from a file; update the router's firmware; and so on. It's always helpful to back up the router's settings before you make changes.
You'll find a lot more settings and features on a router's web interface, and when have time, you should try them out. If worst comes to worst, you can turn to the last-resort step below to restore the router to its default settings.