What's the difference between 33 1/3 60 vs 50 Hz?

This question comes from a previous post concerning a free strobe disk. At the site there is a 60 hz and a 50 hz version. I thought the hz is the # of AC cycle or at least is a measurement used for electrical purposes. The 33 1/3 is RPM's. Why are these measurements of 2 different entities used to make 2 different strobes. I understand why you need a different one for 45 or 33 but don't understand the difference that hz makes. Can someone explain this simply?