There are many way to create a bootable USB drive. Not every method will suite your needs depending on what you’re trying to do. Chances are if you’re trying to copy a Windows DVD to a USB then the Windows 7 USB/DVD Download Tool should be all you need.
If you’re trying to create a bootable USB drive to use with a Linux install CD/ISO or create a multi-boot USB drive then you should check out Pendrivelinux and the YUMI tool.
I find myself most often making a bootable USB for booting into DOS to flash something. Whether it’s a BIOS for a motherboard or a firmware for some device. The easiest and quickest way I have found is to use Rufus.
To create a bootable USB drive with Rufus just insert the USB drive into an available USB slot and open the Rufus exe. Make sure you select the correct device in the first drop down. Check Quick format and Create a bootable disk using: FreeDOS or MS-DOS. Click Start and the USB drive will be wiped and partitioned as a bootable device. You can then copy your files to the remaining free space on the drive.
When you reboot your computer you may have to enter the UEFI/BIOS by tapping or holding down the delete key, F1, F12 or another assigned key to be able to select the USB device as your boot device. Most modern motherboards give you an override option to boot from a selected device in the exit tab or menu.