“YUMI (Your Universal Multiboot Installer) can be used to create a Multiboot USB Flash Drive containing multiple operating systems, antivirus utilities, disc cloning, diagnostic tools, and more. Aside from a few distributions, all files are stored within the Multiboot folder, making for a nicely organized Multiboot Drive that can still be used for other storage purposes.” (source)
First of all, you should know that YUMI is a Windows application. When you launch it, it offers a nice list of Linux distributions. It can also download the ISO files. Just specify your USB stick and YUMI installs the distribution on it.
Ubuntu has its own startup disk creator, you can find it at System -> Administration -> Startup Disk Creator. I used that some times and if I remember correctly, first it erased the USB stick. So with this one you can put only one distribution on a stick, while YUMI permits several distributions on the same stick.
With YUMI I put Ubuntu on a stick. When I booted the stick, I got a live version of Ubuntu. It’s not a full installation, which means that when you reboot the machine with the stick, you lose all your changes and you get a virgin Ubuntu live again.
I’d like to put a normal (not live) Ubuntu on a USB stick. I’ll have to figure out how to do that.
A friend of mine asked me to reinstall her laptop and put Windows 7 on it. I had Windows 7 in ISO format and I didn’t want to burn it on a DVD. With YUMI I could put the ISO on a USB stick and I could install Windows 7 from it without any problem.