It takes a given frequency and divides it into a lower frequency.
This one is a divide by 100, so if you have a 40MHz signal, out comes a 0.40MHz signal. It can also be made to be divide by 10 or both.
Reason for making one: You can get by with MUCH cheaper frequency counters and similar equipment, and work with MUCH higher frequencies. Also you can clock multiple devices at different speeds keeping them in sync.
Pros:
Cheap
Works well
Readily available parts (7490N or 2).
Easy to make
Cons:
Produces a square wave output (so it's best for counters and similar)
32 to 42 MHz max
