You can use a microphone and computer speakers or
use a headset/microphone. Either way, you will need a speaking device and a listening device. For the headset you will see two jacks (perhaps red and gray) at the end of the wire leading from the headset. These two jacks need to be inserted into your system unit.
OR
You can purchase a USB headset with microphone that can be inserted into a USB port. "Short for Universal Serial Bus, an external bus standard that supports data transfer rates of 12 Mbps. A single USB port can be used to connect up to 127 peripheral devices, such as mice, modems, and keyboards. USB also supports Plug-and-Play installation and hot plugging. " (http://webopedia.internet.com/TERM/U/USB.html) .
You are required to have a microphone and speakers for your Elluminate Classes.