A hertz is defined as one cycle per second. In a PC, the speed of the CPU and the speed of various busses on the motherboard are measured in megahertz. For example, a 486DX-33 CPU chip is rated to operate at speeds of up to 33MHz. This is normally indicated in nanoseconds. DRAM chips, such as on a SIMM module, typically have an access time of around 70 ns.
Hard drives have an access time rating which is usually expressed in milliseconds.
This ranges from 200 ms on older drives to 10 ms on newer drives.