DFU = Device Firmware Update. Only use DFU mode as a last resort, if a restore using Recovery Mode doesn't work . When you enter your iPhone into DFU Mode it does not load the OS before attempting the restore therefore it is able to Unbrick your Bricked iPhone(it will work 90% of the time).
Open iTunes and connect the iPhone to your PC or Mac.
Press and hold the Home button and the Sleep/Wake button at the same time.
After exactly 10 seconds (Count 1 thousand 2 thousand 3 thousand 4 thousand........) release the Sleep/Wake button. Continue holding the home button until you iTunes pops up a message telling you that it has detected an iPhone in recovery mode.
The iPhone screen will remain black like this:
If anything else like this:
Try again ! Make sure get your timing right.