Routing using second interface

view story

http://unix.stackexchange.com – I've got a machine I control running Ubuntu 12.10. I'm on a network that will only allow me to have one device connected. I've also got a USB ethernet adapter. I'd like to create a private network hung off that for eg my Raspberry Pi so I can SSH it and it can access the internet. Can someone please explain the concepts of what I need to do in order to achieve this without mucking up my main network connection? Not knowing exactly what it is I need to do in the vast field of linux routing has made googling somewhat futile. (HowTos)