running time measured by clock cycle Some literature said the running time of attack algorithm is measured by clock cycles. What is the meaning of clock cycle and why  the running time of attack algorithm is measured by clock cycles?
Some literature said the running time of attack algorithm is measured by clock cycles.


I suspect that either you misunderstood what was written (sometimes we measure the speed of crypto algorithms in terms of clock cycles, because that is somewhat more platform independent than, say, time), or you have taken it out of context.
  I'll assume the literature was discussing the performance of a cryptographical algorithm, rather than an attack (which we generally do not feel the need to define the performance as tightly).

As for what a 'clock cycle' is, well, when we have a computer (CPU), then generally the CPU executes instructions based on a master clock (which runs, at least for periods of time, at a fixed rate); the various operations that the CPU performs to execute instructions are based on this master clock.  One 'beat' of this master clock is termed a 'clock cycle'; that is, if the instructions used to encrypt a byte of data in completed in 10 beats of this master clock, we say it runs in 10 clock cycles.

Of course, it turns out that real computers are rather complicated affairs, and so the above description is both vague and simplified.  They sometimes have multiple independent clock implementing different parts of the processor, and sometimes the rate of the clock will vary (because of a number of reasons).

What 'clock rate' can be used to compare the efficiency of two different algorithms on the same processor, as it (mostly) factors out the differences between absolute clock speed; for example, it can be used even if one processor is running at 2GHz and the other at 3GHz.  However, comparing 'clock rates' between different CPU types (for example, Intel and ARM) makes absolutely no sense; in fact, it makes little sense to compare clock rates of different Intel internal architectures.