Ubuntu on USB stick?

Hey Guys
I have an Ubuntu CD and I was thinking of creating like a bootable hard drive with various OS so that I can just boot OSs with t drive and not require the CDs. I was just wondering is there a way I can do this, like have Ubuntu boot from a USB stick? If yes how is that possible(even if I can just have Ubuntu on one stick, how can I use my USB stick to boot it)?
Also can I create one on an external hard drive?is it the same thing?

Result from googling for ubuntu usb boot

An easy way is to download the UNetbootin software and make one - UNetbootin - Homepage and Downloads

Unetbootin(Universal NETBOOT INstaller) is the best option IMO