Driver is a program that controls a device. Every device, whether it be a printer, disk drive, keyboard, or soundcard must have a driver program. Many drivers, such as the keyboard driver, come with the operating system. A driver acts like a translator between the device and programs that use the device. Each device has its own set of specialized commands that only its driver knows.
In audio field, we have many drivers for soundcard. Name it MME (the oldest Microsoft generic driver), WDM (Microsoft's newer generic driver), ASIO, GSIF, etc. It's which card supports what driver on what application that we should know.
To make it short, I heard EMU cards doesn't support WDM (?). So in order to use it in Sonar, you must choose either ASIO mode driver, or MME driver. ASIO works alot better than MME. When you choose ASIO, there's a chance you won't see your onboard sound card on the list anymore (until you switch back to MME mode) since your onboard soundcard doesn't support ASIO. So you'll see only the EMU on the list. Select them to use. You'll also need to "tweak" your soundcard's setting from it's Emu control panel.
Jaymz