A bootable USB in simple terms is one from which a computer can be able to start or boot without having to use the hard disk. It can contain an operating system or can be a live USB drive.
Having knowledge of making a USB Drive bootalbe can come in handy especially when you want to use a live CD or to install an operating system to a computer which has no CD/DVD drive or one with a faulty CD/DVD drive but has working USB ports.

Below are the required tools for this tutorial
  1. A flash disk / memory card (Preferably 8GB though a 4GB can also work depending on the Windows 7 image size)
  2. A windows 7 image file (Preferably .iso format)
  3. A windows PC (XP and above)
  4. Power ISO (This can be downloaded from here )
Steps to create the bootable USB drive 
  1. Insert your USB drive (Remember to copy off your important data as the drive will be formatted) .
  2. Open power ISO with administrative rights (Right click power ISO icon and select run as administrator).
  3.  When power ISO opens, click Tools then click Create bootable USB Drive and window will open, browse for the Source Image File which is your windows 7 image
  4. Then select the USB drive to make bootable
  5. LeaveWrite Method selected to USB-HDD
  6. Click Start to begin creating the bootable USB drive
  7. Once the process completes successfully, then your USB drive is bootable and can be used to boot any pc that supports booting from USB
Below are sample screenshots