Robert Hansen: Timing attacks are when you use logic of the logic of a website against itself to reveal data. The precision of a clock is really for human benefit, not for the computer s. In reality, when you look at the clock on your computer it is only showing you the significant bits that you are. In reality it has a much higher precision than what it shows you. When your browser makes a request, the server will return the time. When for instance you type in a correct username but an incorrect password it could be that the server makes two requests instead of one. But if for instance. you type. in an incorrect username, the vulnerable server will never bother making the second database request.
Source: https://threatpost.com/gaining-precision-information-leakage-attacks-063010/74165/