Lets take an example to explain it perfectly
You see that windows 7 recovery iso file is bootable but when you copy it to pendrive the pendrive does not boot; so to make it bootable you have to use an extra tool e.g. xboot
Link -
sites.google.com/site/shamurxboot/download
1st drag that iso file to opened xboot's middle portion then click create USB
CLICK PICS
[IMG=http://img28.imageshack.us/img28/4905/captureejt.png]
[IMG=http://img830.imageshack.us/img830/9536/capture1ju.png]
Download:
https://sites.google.com/site/shamurxboot/XBootv1.0beta14.zip?attredirects=0