I've been playing with the depth of field settings lately, having great fun with them, but can someone explain what the shutter and f-stop settings do, or change?

I tried lowering the f-stop on a test render, and took pity on my laptop and killed the program as it churned and churned...

So what's the difference in f-stops for? Shutter settings?

TIA as always to the Poser geniuses - Martin