I would like to know the difference between an ADSL Modem (USB / Ethernet based) and an ADSL Router.

I am using Windows Server 2003 and my Broadband provider gave me an Ethernet based ADSL modem and it worked fine. All I had to do is switch on the PC and I was live on the internet.
Now the company has installed an USB modem which does not have drivers for Windows Server 2003. Even though it connects at times, I am not Live on the internet as soon as I switch on the PC. I have to do some dialing with username and password and it gies an error. Whatever it is, its a driver related problem.
The Ethernet based modem had no drivers to install. All you have to do is set the LAN IP and DNS IPs an you are Live..

Whats the difference between the two modems ??
and By the way And what is an ADSL Router ??