What is clock speed and clock cycle?

Clock speed is the term that refers to the number of changes in voltage in one second. For instance, 1 GHz means that the actually voltage across the component in question has a power supply that switches from 0 volts to 5 volts, and back to 0 volts, 1 giga (10^9) times every second.

Clock cycle simply refers to a single transition from 0 volts to 5 volts and back.
Clock cycles refer to the central processing unit's processing speed. It's measured in hertz -- current processors have clock speeds in the gigahertz range.