How do I setup Ubuntu interfaces so one interface is "mapped" to another?

I have recently setup a DHCP-Server on an Ubuntu 10.04 server machine. The machine has two interfaces (eth0, eth1). I have the DHCP server setup correctly because computers connecting to the network get a DHCP IP address but they cannot connect to the outside world.

I believe it has to do with the two interfaces being mapped/bridged together. How do I properly map one interface to the other? Is it bridging or mapping?

I see on this page: https://help.ubuntu.com/community/dhcp3-server they do something like

mapping hotplug
    script grep
    map eth1

What does that exactly do?

In other places I see the following:

iface br0 inet static
bridge_ports eth0 eth1

Which is better or more efficient?

Sorry for all the questions, I'm a noob at setting up a DCHP Server.

Thanks for your help.

EDIT:

This question is in relation to a previous question I asked on ServerFault: Is it possible to setup a DHCP Server for localhost only?.

Basically I want to setup one interface (eth0) to connect to the outside world while (eth1) listens for DHCP requests from the VM's (bridged network setting in VMWare). It's setup on a server and the VM's are properly getting the DHCP IP's but they cannot see the outside world. :(

I used the configuration files from this post as a baseline to setup DHCP http://ubuntuforums.org/showthread.php?t=825294 (only difference we have 2 interfaces while they have 3) but unfortunately they didn't share their interfaces setup and its an old post.