A bootable USB drive is a stand alone drive that a computer can boot from or use to install an operating system. If you have ever needed to install Windows on a new computer, this is the way to do it.
A tool called Rufus for Windows allows you to burn .ISO images to a drive and makes it bootable. There are various images that you can download including Windows, Linux, Mac etc.
For demonstration purposes, I have an .ISO for a popular disk partitioning software (GParted) which I am going to boot as a Live CD. This allows the computer to run directly from the drive with no installation. After inserting a flash drive with a high enough storage capacity and backing up any data on it, you can open the rufus.exe file.
After this, select the USB device from the Device dropdown section. Choose your Partition scheme and target system based on your computer’s BIOS. Name the volume label and then select your ISO image. After you have done it all, you can click START.
If you get a message like this, select ISO image mode and click OK.
Be sure you have no files that you need and then click OK because all your data will be erased.
After the bar is completely green, shutdown your computer and then click the F8, F11, or F12 button as it varies. Try all of them to see which one works. Then select your newly created drive from the screen.
Once you do, you are free to do whatever you want in the Live CD environment or to install your OS.
This is a very useful tool and can be used for troubleshooting your PC or to use special software that needs to be run directly, not on top of an OS.