Slush score system, please explain

[dellnull]

I'm on slush's pool and as a newbie as I am, I can't figure how the score systems works. Can anyone give me a hint? I have one GPU and 3 CPU and the scores make no sence to me. It looks like that with a shorter time since last share = higher score. So with a slow CPU miner you don't get new shares in a quick steady stream and therefor your score decreases?
And how does the score affects the the reward?