Most modern computers will allow you to select specify the "boot device" by pressing a function key—usually F1, F11, or F12—as your computer is starting up. Jamulus OS is a complete Linux operating system written onto a bootable USB thumb drive. You insert the Jamulus thumb drive into a USB port, press the appropriate function key as the computer starts up and select the USB drive as the boot device. Instead of your normal operating system's desktop, the Jamulus OS desktop will be displayed, allowing you to run Jamulus without having to install it on your work-a-day operating system. It's like you have a new computer, with Jamulus...and a host of other professional level media production tools available.
Jamulus OS is provided as a downloadable file which contains an "image" of the working operating system copied from the developer's hard drive. You can find that original file here—Jamulus OS Wiki—along with instructions on how to copy the file onto your own USB thumb drive and make it bootable.
There have been an number of updates to the Jamulus software since this ISO image was created. There are utility programs (called scripts in linux) provided to help you update the Jamulus program originally installed on the Jamulus OS. It is very advisable to run these, as some significant improvements have been made to both the GUI and the core software recently.