Computers use an internal clock to synchronize all of their calculations.
The clock insures that the various circuits inside a computer work together at the same time. One Hertz equals one tick per second. One Megahertz equals one million ticks per second. A 166MHz computer will be faster than a 120MHz computer with the same processor and the same amount of memory . However, clock speed can not be used to compare computers with different types of processors or computers with different amounts of memory.