What is the difference between an i5 CPU Core and an i7 CPU Core, if both of them are rated at the same Hz? Which is better for regular office work?

the main difference between the Intel i5 and the i7 is hyper-threading.

hyper-threading is a technology developed by Intel which allows each physical core to be utilized as two logical cores in the operating system.

i5 = 4 physical cores

i7 = 4 physical cores + 4 logical cores which the OS will see as 8 cores.

for normal office work an i5 is more than sufficient but in applications that benefit from more cores like video editing or 3d rendering the i7 would be a better choice.