If you want ADSL for the office, go with Ethernet.
Ethernet is basically the same as your LAN network - the same RJ45 connectors that look like big telephone cable plugs.
For the office, you would get yourself an ADSL Router with either 1 or more ethernet ports.
This you would hook up to your LAN network switch and then access it via a web-browser to configure it - all that info will be in your ADSL Routers manual.
USB is where you hook your ADSL modem directly to a computer via a standard USB port, such as what your mouse uses.
imho, it's not as useful as Ethernet, but it's a lot easier to configure - more or less Plug and Pray.